Dig around in the logbooks and find names and certificate numbers of folks who have worked on or flown the aircraft. A quick search of FAA records will provide an address....do a bit of digging and have a chat with them and see what they remember about the aircraft. In shoddy outfits there will be ample turn over and some number of folks that would love to participate in a bit of Truth telling. Use those conversations to look for problems in documentation and the like. If you are talking about a 412.....you can afford to do a real research effort considering the down side of buying an aircraft that is not as advertised somehow.