tricky as some red flags start out very tiny and only begin to unfurl as you get attached to them. Lots of wise words, one that hasn't been mentioned is if he goes too fast, acts romantically very soon, says how perfect you are, contacts you very often, wants to be with you all the time, has exactly the same interests as you, goes on a real charm offensive and gets you hooked, and then he can drop the charm bit. a man like this can sniff out a needy woman. You sometimes see stories about older British women who go out to a poor but warm and sunny country on holiday and meet the love of their life, a young charming man who falls madly in love with them, and when he has basically got what he wants, money, whatever, will disappear from the scene. Not saying it might always be the case, but that is one end of a spectrum, where it is obvious to everyone but the poor fleeced woman, not all are so obvious
Decide what your deal breakers are. Does he resent you seeing your family, that would be a deal-breaker for me for example, is he stingy, penny-pinching beyond reason, or is he so generous that unless he's a millionaire there will be a problem with this down the line, your money/house will be at risk, what would you have to give up if you started a relationship and what do you not want to give up, hobbies, friends, family, does he think you ought to sell your car as you don't need 2, then you are reliant on him for example? Does he basically just want someone who will look after him, so basically wants a housekeeper or will he be an equal partner. Someone said an equal financial footing, as you get older this is even more important I think. Is he over-reliant on alcohol, what are his previous relationships like, does he diss all his previous partners or bosses - chances are while some might have been unreasonable, they were not all totally unreasonable or crazy. And how does he treat other people, those in service roles for example. Is he a racist, are you politically compatible, doesn't mean you have to agree necessarily but if you are living together you don't want it grating on you every day.