If they're truly crazy, you'll never know it from screening. Now if they're batshit crazy, you might pick up on it. You've probably seen more crazy people than you could possibly imagine.
You really should get on P411. At least you can see if the prospective cleint has multiple OK's, which Providers gave the OK's and if the prospective client has any black marks. Not foolproof, but better than most anything else.
