Well, I'd start with checking whether the person has a history of suing or being sued (e.g. by stiffed subcontractors). Also whether they take apparent pride in that history.

Further, ask people he's been in business with in the past not about him but about whether they'd do business with him in the future.

This is pretty good advice. The person who sued me had also sued other people. If your local court(s) have a searchable online database, search for the person's name and/or the name of their company.

I'd also recommend trying to not putt yourself in a position where you can only pay a subcontractor when you get paid. When you're client is late on payment, and your subcontractor starts hounding you for cash, you won't have a good time.

Asshole is a pretty subjective measurement. Warning signs are people who push you around, rush into getting a contract signed, ask too many questions for a really small contract, try to control details of how you'd do the work, etc. A lot of it you learn by experience.