In the six years I have been doing residential/commercial HVACR, I have never once run into a job where I couldn't find out what was charged into the sealed system. That is to say, I have never seen a refrigerator, freezer, heat pump, RTU or air conditioner that did not have the information listed on the manufacturer data plate.
If the equipment was originally charged with R12 or other refrigerants but was recharged with something else, or if the sealed system was replaced entirely (not common or cost effective in domestic fridges, but possible with central AC/heat pumps and virtually all commercial HVACR equipment), the technician is supposed to affix a label near the original data plate with the type of charge and maybe a date, or at minimum cross off the original specs and rewrite the changed ones in a Sharpie.
If the technician failed to do this, and I'm sure this is a problem because there are hacks out there, clues are the next best option. Look to your compressor and its data plate. This is not accurate if the equipment just had an evac/recharge, but simply if the compressor was replaced at some point. You won't be seeing barcodes on the data label of an OE compressor installed in, say a '60s Coldspot or '50s Vimco-Victory.