How (sometimes) to tell: If it's a subject that one person knows a lot about, ask the other person questions about that subject. If they answer the questions correctly/immediately, their cool, if not.........LIAR!
When I was involved in rodeo, as a Team Roper, I would talk to different guys at local nightclubs about the event. It didn't take me very long to figure out the guy wasn't a roper and/or knew very little about the event. I'd ask them the brand of rope they used, the type of rope (scant), where they went to weekend Jackpot Ropings at, whether they were a "Header" or "Heeler" and perhaps the name of their partner. For those that answered correctly, the conversation went on, for those that stumbled around answering, I'd politely excuse myself from the conversation or they would say "hey listen, I gotta go".