I was 'caught' once by someone on online dating who was looking for a little extra-curricular activity, shall we say: he had an extra phone with a pay as you go sim card in and he even went so far as to give me a fake name, so all the checking up in the world probably wouldn't have helped save me much beforehand. In fact I was so oblivious we had dated about 5 or 6 times before I discovered it - and that was only as he was going to pay for dinner (he was very chivalrous like that, did impress me as I had an EA ex who has left me almost bankrupt) and I saw his driving license in his wallet, went to grab it jokingly to see the pic and he suddenly got all weird, snatching it away from me and being all snappy and weird. Even then I didn't actually twig but he confessed that he had given me a fake name and eventually let me see it (whereupon i found he'd been shaving about 8 years off his age as well!)
Even then I must admit I believed his excuses which were that he had a very difficult relationship with his ex wife hence was keeping his online identity secretive …. the truth was that his wife wasn't his 'ex' at all.
Now I am getting a bit wiser although still bloody damn gullible but my best advice to you is to just totally trust your instincts. This guy was pretty intense straight away, which I did find strange but got pulled in because I was flattered, but in retrospect it should have been a red flag, he barely knew me. [I think the cheating element makes them feel more intense maybe?]
couple of practicalities;
let him pay for something (even just a round of drinks, not ness dinner etc! up to you ) does he pay in cash always, or use bank cards? if he never gets out a card, or goes to a cashpoint - or doesn't have any id on him - be wary
is he open about his phone? (checking messages is rude so this is a tricky one but if he gets a call and ignores it in retrospect I might be slightly suspicious. But hard as he could just be being well mannered and giving you all his attention!)
if you feel a bit unsure, be honest, say you've been trickstered in the past and just ask… a decent, open guy won't mind, but someone with someone to hide might give himself away but acting affronted/angry/unnecesarily combatitive
i've still got a long way to go (my bullshit radar is not very good) but just be self protective - most of the things I suggest would probably only emerge after a few dates though, so get the first one over with and see if you even like him before worrying too much
good luck … and have fun!!