A problem is, some things they don't let you know until you get up the duff. I remember being about 4 months pregnant and my ex announced his mother was an alcoholic and he wouldn't trust her alone with a pet rock. I'd met her, spent time with her, thought she was lovely. Turned out whenever I was due round a massive operation took place to keep her sober and pretend the family got on with each other. After I was pregnant that gradually faded. The massive rows then happened in front of me.
Also pay attention to the father son relationship, there can be a crazy amount of hidden baggage there.
I don't know if these things can be spotted, and I think it's cruel when the family work together to try and keep the gf around. If left to his own devices I reckon my ex would have been easy to spot. His home would have been a shit hole, he would have been out of a job, he wouldn't have been able to afford to go on dates etc. If your son is a dick let women see that and save everyone heartbreak. Don't wait until they're 8 months pregnant and then out of the blue tell the poor woman actually you think your son is a really bad person.
Pay attention to 'banter' even when he says 'oh that's just what Jim is like, I haven't the heart to tell him I don't like his sexist jokes'
Pay attention to relationships with sisters.
See how he reacts when you want to choose things, pay close attention. Is he enthusiastic about your choices, does he try to make that happen, or do your choices always seem to be unable to happen for one reason or another and you always end up doing what he'd suggested.
Also, might be controversial, if he has ever asked to borrow even a small amount of money while dating or told any single lie (no matter how small/white) it would be over immediately now. No benefit of the doubt for me anymore. I don't care if a tiny tornado appeared out of nowhere and stole only your wallet, I don't care how kind you thought you were being when you lied, you're not in my life anymore after that point. Because that gradually became big lies and lots of money, ever so slowly.
I think it goes without saying that if you're getting to be choosey about partners don't start dating men who already have children. It's an easy way to never have the hassle of being a step mum if you consider those men ineligible from the get go.