If you are a programmer, you can tell; just ask them about their previous projects. Dig into their answers and ask follow up questions. What data structure did you use to do this part? What in the database was designed poorly and why? (Every database has a design issue).

I mean if you were at a party and some guy came up to you chatting, claiming to be a senior whatever at BigTechCo, you'd start chatting him up and you could tell if he was full of BS pretty quickly.