I would have to agree with you here. Photos of the coins perhaps, if he gets funny then that is a sign!
Also I guess if you get out some of your own scales, measures and calipers and he got funny that is a tell tale sign!
Asking for his credentials and business card or contact information seems a legit thing to ask too which might provide warning signs.
Sorry, but that's hardly a fair way of telling if gold is real.
I sell face to face sometimes, and if someone wanted to take pictures of me there would be no deal - and I would never deal with them again. Some people value their personal privacy and security more than a sale.