From my experience and mistakes -
Can he discuss a problem?
Does he shut down and stonewall if you try to talk to him about a difficult subject?
Does he have endless grand ideas which never come into fruition?
Is everything always someone else's fault?
Is it "everything will be fine after [x has happened]" but x isn't realistic or keeps changing
Are all his exes crazy?
Is he careless with money?
Does he help you and listen to you when you talk about what matters to you, or is it just one sided?
Does he make derogatory remarks about women?
Does he have any friends? If no, be very wary, there's usually a reason
Does he seem incapable of ever being happy, or satisfied with what he has?
Do you always end up doing what he wants to do, or going to the places or holiday destinations he chooses?
Does he expect you to go and visit his family, but never comes with you to see yours?
Does he shirk responsibility when it comes to things like looking after a pet properly?
Does he fob you off and say he doesn't know what he's looking for right now, let's just see what happens?