have a look at the paper " The use of buccal swabs as a minimal-invasive method for detecting effects of pesticide exposure on enzymatic activity in common wall lizards" from Mingo et al. (2016).
They analysed GST, GR and AChE in wall lizards. I would guess that this will also work for amphibians and several other biomarkers. We took some Bufo bufo saliva samples to test this last year, but haven't analyzed them yet.