Aside from using a refrigerant identifier, one can identify the refrigerant by measuring the pressure on a stable system (one on a car that hasn't run in 12 hours) and absolutely measuring the temp (the reason for a stable system). Then comparing the results to temp/pressure charts for possibly used refrigerants.
As an example, R12 at 85deg F would be under 99.8psi. At the same temp R134a would be 103.9psi. This is a way to tell what is in the can. If checking a 30lb cylinder the pressure will be according to these numbers as long as there is liquid in the can. It doesn't matter whether it is 2lbs or 30.
Bosch Master, ASE Master, L1
33 years MB technician