The single most important thing to watch out for is the relationship that he has with his parents/caregivers especially his mother. Our first relationships are with our parents or primary caregivers. This is the model of a relationship that will go forward into his adult life. 'Good enough' parents are firm but kind; have appropriate boundaries; model reasonable behaviour and supportive relationships.
It's true that there are quite a few men around who don't like women much or don't respect them and this will almost certainly relate to attitudes and behaviours picked up from parents. I didn't have sons but many of my friends with sons admit that they spoil them, over and above their daughters. I was shocked when one of my daughter's boyfriends lived with us for a time as I definitely fussed over him and my daughter found it irritating! It was partly the novelty of having a boy in the house but I honestly shocked myself with my behaviour. The boyfriend loved it of course!
If the parents are too indulgent and lax about poor behaviour, boys end up not taking much responsibility for how they behave. They expect to be indulged by adoring women even if they behave badly. If the mother and/or father don't set standards for decent behaviour then there is no real incentive to change and he will expect his girlfriend/wife to do the same.
The other thing to watch out for is empathy. There are a lot of narcissists around (and other personality disorders) and by the time they reach adulthood they have learnt to disguise this quite well. I was married to a covert narcissist - he hid it very well, but the sense of undeserved entitlement was definitely there and still is. I think he lacked depth of genuine emotion. Look for genuine emotion and someone who has a strong sense of values - i.e: good character.
Sadly, getting all that along with a man who you also find sexually attractive, and vice-versa, interesting, intelligent etc is rather like searching for the unicorn! Sometimes, though, good things come in unexpected packages.
Totally agree about not ignoring red flags at the beginning. Every single red flag I ignored were the things that in the end broke the marriage. Every single one - they were all there practically from the word go and I also found that once we were married he turned into a version of his father with a huge sense of entitlement. So watch out for how his father treats significant women in his life because this is a boy's role model for relationships. That was another red flag I ignored. On my first visit to my ex-husband's house, his father didn't bother to get up and greet me but remained slumped in a chair in the kitchen. His father was more of an overt narcissist. Manners do matter as they demonstrate the degree to which someone will behave in order to make a good impression. Of course, manners can be faked so you have to look beyond them but they still matter.