AGAIN, there is NO WAY to know what's in there without a leak detector. If everyone followed Federal law which requires correct fittings for the particular refrigerant and a sticker with pertinent information then you would know. That said, I would be surprised if as many as HALF of the cars that have refrigerant other than original are properly labeled and have correct fittings.

If it were mine, I wouldn't even DREAM of taking any chances. I would sniff for leaks first if there's anything left inside. Then I would break all connections, flush all components, replace the r/d, add the correct type and amount of oil, evacuate and charge by weight with the original refrigerant type. BUT, that's just me.