Purchasing used...how to know if it is stolen or not?

same question could apply to buying a used washer/dryer or a used lawn mower. None of these are vehicles either.
Unless you are in a State where the TC is considered a vehicle and has to have a tag there is not a lot you can do, but also not a lot that can be done if it is stolen unless the owner actually sees it and starts legal process. Then he will need proof that it was his.

Not sure if you are worried about the morality of buying stolen, or of the possibility you lose it.