I am working on the connection between foreign language learning anxiety and L1 identity, and want to know if we can measure feeling of or attitude towards L1 identity, especially at the adolescent level.
Generally speaking, measurements whereby human perception or interpretation is an important factor have long been used as a basis for decision-making in different qualitative studies. The following links can hopefully satisfy what you are looking for:
Article Language attitudes, identity and L1 maintenance: A qualitati...
language attitudes tests may be the best way to test L1 perception . One may also conduct interviews with l1 speakers in which different issues related to their languages are discussed.