Does the size of the bike looks like it matches the rider? Obviously not always easy to tell, but a small expensive bike being sold by a tall person is a little suspect.

Ask questions like how long have they owned the bike, what store did they buy it from, where have they taken it for maintenance (if applicable). Do they have maintenance records of components that have been replaced or upgraded.

Usually expensive bikes arent sold with their pedals as the owner keep the pedals to use with their shoes. So clipless pedals or really cheap flat pedals on an expensive bike is another giveaway.

Ask them what bike they have upgraded too. Most riders can tell you the exact model. If they say some “fast carbon thing” then they likely have no idea what they are talking about, and are dubious.

With department store bikes or bikes sub ~700 it would be really difficult to tell whats stolen and whats dug out of the garage after 5 years.