14N NMR. Somewhere -350 or so is NH4+ , ammonia is -300 I think, and amines are between -100 and -300.
Do do 14N of compound and precursor.
If you see a peak in the desired range, dope the tube with a mix of NH4X and NH3(aq) to make sure the observed peak is not ammonia or ammonium.
Or mass spec.