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.

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Steve Brotherton
Continental Imports
Gainesville FL
Bosch Master, ASE Master, L1
33 years MB technician