Yes it SHOULD have a sticker and conversion fittings, but there are zillions of systems that have hydrocarbons, blends, sealants and who knows what in them. If there is a sticker that's great, but if there isn't one there is still a good possibility that there is something besides R12 in there.
People have no idea what grief and problems that they are causing the a/c repair people with all the junk refrigerants and sealants that they are putting in their systems.
This junk is contaminating recovered refrigerant, clogging up expensive recovery equipment and just making general H$!! for those in the business.
It has gotten so bad, that most shops put their seal on the ports after service. If that seal is broken when the car returns, they will not do ANYTHING to the system before analyzing it again with their Refrigerant Identifier.
Have a great day,