Not all turbos had the decal, there are a few ways to tell if it is a factory TJ or a conversion (there are folks on here that know waaaaaaay more about this stuff than I do, y'all feel free to correct me!)
I believe the factory turbos had the H16 style dolphin striker, where the conversions had the triangular striker with the holes in it. If it is a well done conversion (should be super tight with zero play) I wouldn't worry about it. It all depends on who did the conversion.
Another way to tell is the age of the boat...the TJ was not offered until 1982. If it's a pre-82 boat, it's definitely not factory.
Having said that, I would be more concerned with the integrity of the hulls than anything else...several helpful threads RE: delamination on this forum...and bottom jobs. If the hulls are shot keep looking. You don't want to spend your first summer as a cat sailor working on your boat.
If possible, have the owner rig the boat for you...make sure it's a complete boat.
Be patient, this time of year boats for sale come out of the woodwork. I would buy the absolute best boat that you can afford to buy. A $1000 boat that's ready to sail today is a better deal than a $500 boat that needs a trampoline, main halyard, new rigging, trailer lights, etc.
83 Hobie 14 Turbo - Cat Fever
86 Hobie Holder 12 "Mello Yello"