It's interesting, you'd think that a concept that popular would have established measures, but the only questionnaire that popped to mind is Stephan & Stephan's questionnaires on integrated threat theory. You might want to look into that. 

A quick search suggests that "social identity threat" in strict tajfelian sense is more often manipulated, not measured (see Steele). What they say is:

"explicit threat measures rely on the assumption that participants are subjectively aware of the threat that occurs, while this is not at all self-evident (Blascovich & Mendes, 2000). Furthermore, selfreports are open to reactivity and defensive reactions, leading those who are most threatened to indicate this to the least extent  (Bettencourt, Miller, & Hume, 1999; Branscombe, Ellemers, et al., 1999; Branscombe, Schmitt, et al., 1999)." - quote from Scheepers & Elleners, 2004. The authors argue for using cardiovascular measures of social identity threat, and, on the first glance, show actual results. 

This argument makes certain sense intuitively, but a decent literature review would be required to see how difficult it actually is to measure social identity threat. 

Overall, if Stephan&Stephan's questionnaires suit your needs, I'd use then, they are tried. But a good literature review on dangers of measuring your construct is still in order. 

hi,

here are the publications :

http://psycnet.apa.org/journals/psp/67/2/243/

http://www.sciencedirect.com/science/article/pii/S0065260102800090

http://psycnet.apa.org/journals/psp/94/4/615/

http://psycnet.apa.org/psycinfo/2009-07435-001

Dear Viladimir,

Thanks a lot for your detailed and comprehensive response. I went through Stephen & Stephen's (2000) integrated theory concept. It is really interesting. Don't you think it is a more holistic concept and encapsulates the idea of "social identity threat"? Or can we treat them equally?. I would really like your input on this. Thanks a lot once again,

Dear Dr. Rahimi,

Thanks a lot for your response. I have started reading these publications. Infact, there is an "identity threat" scale designed by researchers in the first link provided by you. Thanks a lot for your guidance. 

Dear Salman, 

I am not an expert in intergroup relations, so I am a bit humbled by a request to express opinion on which of the major established theories makes more sense. I consulted with a colleague who is more knowledgeable. Here is what we came up with:

1. Stephan&Stephan's approach is a bit broader than just "social identity threat". They distinguish realistic and symbolic threat, and also include stereotypes and intergroup anxiety in their framework. So this seems to be a good instrument to address your needs.

2. My colleague agreed that measuring identity threat in a survey is prone to a number of methodological difficulties (see the passage quoted above). People's responses can be biased. 

3. If there is a way to manipulate a measure (do an experimental study instead of a survey study) - that would be preferable in most contexts. Experimental studies are easier to publish, they allow for causality interpretations, and you can try to avoid some of the problems from point (2) that would have been impossible to avoid in a survey. 

Hope this is helpful, and best of luck with your project!

Dear Viladimir,

I am grateful to you for your detailed answer. It was really helpful. 

Thanks Vladimir for introducing identity theories .

I have profoundly  elaborated on them in this keynote talk , probably it could add some points:

https://www.youtube.com/watch?v=Ps4gCvJb6SA&list=PLxS9Z0X37qZpbJkC9hALA_krcSWTx3dRd

Thanks Vladimir for introducing identity theories .

I have profoundly  elaborated on them in this keynote talk , probably it could add some points:

https://www.youtube.com/watch?v=Ps4gCvJb6SA&list=PLxS9Z0X37qZpbJkC9hALA_krcSWTx3dRd

Thanks Vladimir for introducing identity theories .

I have profoundly  elaborated on them in this keynote talk , probably it could add some points:

https://www.youtube.com/watch?v=Ps4gCvJb6SA&list=PLxS9Z0X37qZpbJkC9hALA_krcSWTx3dRd

Dear Dr Rahimi,

Thanks a lot for the link to your lecture. It was very helpful in understanding key concepts of identity theories. 

pleasure salman,  good luck with your research.