There's several questions you can ask to help you determine if its a stolen bike. All these should be asked over the phone or in person, to not let the seller have time to look it up on the internet:

1) Ask for the exact model/year. If the seller doesn't know or has to "look it up," then there's a higher chance its stolen.

2) Ask which store the bike was bought from.

3) Ask which components are upgraded from stock.

These are pretty general questions that any owner should be able to answer right off the bat. If I want to see how honest the seller is, I'll ask how long its been since the chain was replaced, then compare with a chain checker that I bring. If the owner doesn't know, then its probably stolen. If the owner lies, then its either stolen or the owner is dishonest.