I went to a workshop on the treatment of social anxiety disorder with David Clark in July.  It was very helpful.  I've listed assessment & monitoring questionnaires that he recommended below:

As a general measure to assess and track changes in social anxiety severity, the freely available Social Phobia Inventory (SPIN) is the questionnaire recommended by the England & Wales NHS Improving Access to Psychological Therapies (IAPT) "outcomes toolkit".  Interestingly David seems to prefer the Liebowitz Social Anxiety Scale (LSAS) probably because it makes a pretty full job of assessing both anxiety and avoidance.  

Social Phobia Inventory - SPIN - (PDF file and Word doc): a widely used, freely available 17 item questionnaire for assessing social anxiety disorder.  On the second sheet of the download, I give suggestions about what the different levels of score mean.

Liebowitz Social Anxiety Scale - LSAS - (PDF file and Word doc): this 24 item questionnaire assesses both social anxiety and avoidance.  It's good and is widely used in clinical trials.  On the second sheet of the download, I give scoring suggestions and some information about improvement. 

Participants on David's July workshop were subsequently sent copies of four further questionnaires he recommends using.  These measures were emailed as a Word doc attachment in Arial font printing out as "letter" sized handouts.  They are available to download in this format here.  I have also rewritten them in my preferred Tahoma font printing out as A4 sized documents and have listed them individually below: 

Social Phobia Weekly Summary Scale (PDF file and Word doc): this 6 item questionnaire uses 0 to 8 numerical rating scales to assess disturbance/disablement, avoidance, other-focused/self-focused attention in general & in difficult social situations, pre-event anticipatory worry, and post-event subsequent rumination.  

Social Cognitions Questionnaire (PDF file and Word doc): this questionnaire assesses the frequency and degree of belief in 22 thoughts that may go through people's minds when they feel socially frightened or anxious.  There is additional space for writing down and rating less common, more personalized "catastrophic" social concerns. 

Attitudes Questionnaire (PDF file and Word doc): an extensive 50 item assessment form.  The instructions state - "This questionnaire lists different attitudes or beliefs which people sometimes hold.  Read each statement carefully and decide how much you agree or disagree with each one ... Because people are different, there is no right or wrong answer to these statements.  To decide whether a given attitude is typical of your way of looking at things, simply keep in mind what you are like most of the time."

Behaviours Questionnaire (PDF file and Word doc): this questionnaire assesses the frequency of 28 "safety behaviours" that people suffering from social anxiety may perform.  It is suggested that these behaviours often seem like they help the anxiety in the short term, but in the longer term there is a real danger that they maintain the problem.

David also gave us a book chapter handout entitled "The assessment interview and getting started".  Interestingly in the table on "Standardized questionnaires that are useful for collecting information in advance of the Clinical Interview" he mentions the LSAS, Cognitions, Attitudes, and Behaviours Questionnaires (all listed above).  This group of four questionnaires seems to be the standard assessment bundle (plus the Weekly Summary Scale).  They can be partly concentrated down to the Social Anxiety Overall Summary sheet (see below) and then individualized still further when completing the Social Anxiety Flow Chart (below). 

However in the chapter table, the assessment list also includes the Beck Depression Inventory and the 1998 Mattick & Clarke Social Phobia Scale and Social Interaction Anxiety Scale.  Many therapists will have alternative assessment instruments for depression other than the relatively expensive copyrighted Beck Inventory - for example IAPT recommends the PHQ-9.  At the risk of gilding the lily, here are the two Mattick & Clarke questionnaires, which can in some situations provide additional useful assessment, monitoring & treatment planning details:

Social Phobia Scale (PDF file and Word doc) and the Social Interaction Anxiety Scale (PDF file and Word doc): these two 20 item scales assess social situations and aspects of social situations that are anxiety provoking. 

Even if one only uses some of the above questionnaires, there is now a lot of information that has been recorded.  In the book chapter handout we were given, it is suggested that it may be helpful to summarize much of what is most important from this extensive initial assessment on the following sheet:

Social Anxiety Overall Summary (PDF file and Word doc): this form provides a place to summarize information from the various questionnaires under the four general headings - feared situations, avoided situations, negative thoughts (worst fears), and safety behaviours.

Now as a cooperative effort with the social anxiety sufferer, it is recommended that one draws out a "flow chart" of what happens when they become particularly self-conscious & embarrassed.  Many cognitive therapists will do this using a wall mounted whiteboard.  There are advantages to developing the flow chart this way - for example in externalizing the problem and side-stepping what might feel, to the client, an over-intense eye-to-eye interview.  My preference is to sit beside the client and draw out the flow chart on a piece of paper (see below) held on a clipboard.  Using this format one has a sheet that can be photocopied and handed to the client to take away, think about and potentially add to.  Producing the social anxiety flow chart together may well take 40 minutes or so to do well.  At this stage, it is a process of description and "finding out" together rather than an explanation.  Try not to rush the process - when it's done well it can lead to the sufferer feeling very understood.  It is recommended that one select a specific example of a socially embarrassing situation that the client has experienced and can remember fairly easily (a recent event is often good to focus on here).  One starts with a brief description of the situation (top of flow chart).  Moving on to the physical anxiety symptoms that were experienced may well be a good next step (interestingly the Beck Anxiety Inventory may be useful here too).  Then one can enquire and fill in details of thoughts (about perceived social dangers) and sense of self-consciousness (quite possibly linked with an image and/or felt-sense about how one is coming across to others).   One might then move on to producing quite an extensive list of safety behaviours that were involved.  One can add further information from other social anxiety events they have experienced and it may well be useful to give them the sheet to take home and add to after they have been through new socially challenging experiences.   

Social Anxiety Flow Chart (PDF file and Word doc): this is an alternative to the widely used white board diagram. 

Also likely to be useful is a sheet that can be used for recording behavioural experiments - a core component of this highly successful cognitive therapy treatment for social anxiety disorder:

Behavioural Experiments Record Sheet (PDF file and Word doc): this type of homework recording form is likely to be used quite extensively during CBT treatment. 

And lastly in this list, is a copy of the Cognitive Therapy Competence Scale for Social Phobia (CTCS-SP).  To measure therapist skill in treating social anxiety, Clark & colleagues have impressively developed a specific assessment scale - see "Assessing therapeutic competence in cognitive therapy for social phobia: Psychometric properties of the cognitive therapy competence scale for social phobia (CTCS-SP)."  Then, crucially, they have shown that skill measured in this way is predictive of patient outcomes - see "Treatment specific competence predicts outcome in cognitive therapy for social anxiety disorder."  As the authors of the "Treatment competence" paper comment " ... informal use of the cognitive therapy rating scale by students themselves is likely to be helpful. Certainly, we have found that many therapists who are learning CT for social anxiety learn a great deal about how particular procedures should be implemented by studying the particular items on the CTCS-SP and rating their own sessions according to the scale."

Here is the Cognitive Therapy Competence Scale (CTCS-SP) (PDF file and Word doc): a useful way to assess one's own practice. 

For even more social anxiety relevant handouts see this website's "Good Knowledge" page "Social anxiety information & assessment" .