When I was newly raised I had a lot of concerns about knowing whether or not a man I met on the street was a Mason or not. Over time I learned that it didn't really matter. You won't be discussing the secrets outside of Lodge anyway, so whether or not he's a brother doesn't make much difference. If he identifies himself as a Mason and we start chit chatting about fun little stories like brother dfreybur mentioned, that's all well and good, but just like on this site, I know what to say and what not to say.
Regarding how to tell if he's lying, well, over the course of conversation you'll probably be able to tell. A non Mason would probably be nervous or saying things to try to PROVE he's a Mason. A real Mason is just going to complain about the crappy green beans with dinner when all the water/juice got all over everything and made your bologna sandwich soggy. In general, I'll just ask if he's a Mason and which lodge he belongs to. 95% of the time, that's plenty. I've had quite a few experiences where he wasn't a Mason but is wearing a ring or a watch he inherited. Those people have been through it before and will come right out and tell you they're not a Mason. The ones that are Masons will say so and tell you what lodge they belong to and maybe swap some stories. The few who I've encountered who are either imposters or clandestine to the point of being dramatically different will just feel...wrong. And to those I usually just give a knowing look and a "yeah, well, nice to meet you, have a great day" and go on about my business.