If they've been properly converted there will be different fittings on the access ports. They look a little like quick connect fittings found on shop air tools.
That said, this is still no conclusive evidence. There are so many people putting so much junk in a/c systems these days that the only way to know for sure is to go to an automotive a/c shop and have them connect their refrigerant identifier instrument.
By law a conversion is supposed to have the correct fittings and a conversion sticker under the hood. There are, however, many, MANY people illegally converting and putting about every gas known to mankind into these systems.