I'm studying the categorization of oneself as a member of a group (Tajfel, Turner, 1986). Do you know a scale suitable for qualitative research (in-depth interview) and/or quanittative (data collection by questionnaire)?
Most qualitative research using longer in-depth interviews will use questions taken from existing scales and adapted for interviewing participants or they are formulated by the researcher/research team in order to answer the research question. For my recent research in gender identity I looked at the literature around Psychological Androgyny, Femininity, and Masculinity (especially Sandra Bem's work from the 1970s, 1980s, and 1990s) to form questions about personal perceptions and reflections about femininity and the intersection with marital status, in order to explore how these aspects impacted on psychological health and social wellbeing. Therefore I would suggest being guided by your research question and existing literature in order to formulate appropriate questions for your study. I hope this helps?!
You may find this book useful: M. Watzlawik & A. Born (Eds.), Capturing identity: Quantitative and qualitative methods, University Press of America, 2007.
Crocker (1992) a collective self esteem scale: self evaluation of one's self esteem. I was using it a quantitative evaluation of discrimination and social identity
Perhaps you can start with a nominal scale for which you can solicit open ended answers for. Once you have the data you can translate it to qualitative info and measure identities based on answers that are similar in nature.