1) Asking for information that they don't need: Spy's are out to sell your intel, but most are pretty bad at it. Check corp histories. If they are asking for your POS location before even making it in the door, you may have a problem.
2) Asking for access they don't need: Thieves need access, and are often too lazy to work for it. Many will simply ask for access.
3) Leave thief bate. I leave about 50 million worth of ships in the corp area that anyone can access it. If they want to burn themselves on such a low amount, awesome.
4) Look for Red flags: The player is a 2 week old noob, but has their API key in EVEHQ? Might want to keep an eye on them. "New" characters are great because they are nothing to burn. Make sure that noob really is and not some 3 year old player.
5) Prepare for the inevitability: Never put all your eggs in one basket. Don't hold more as a corp then you are prepared to lose, eventually a thief will try for it. Plan ahead, and be ready.