Great question! I think it's really hard to know what the signs of a good/bad father are in advance of actually being a parent. I just looked at my ex playing with kids and thought aww, that would be nice
But anybody can play with kids for half an hour. It's living with them day in day out and managing their lives for two decades through phases where they don't even seem to like you - that's a totally different thing. It's challenging, and above all, you want to pick a partner that will be an asset and complement you through that challenge. Probably the longest and most intense of your life.
How does he treat you when you are vulnerable - sick or tired or struggling with MH or anything like that. Does he take the load off you, show concern, try to make things better for you? Or does he ignore you (or even get huffy). You will be vulnerable for a long time with pregnancy/young children. See also: animals, women in his family (mother/sisters), people serving him (restaurant staff, shop staff, his staff at work etc) Do not look at what he says, look at what he does and how he says things.
Agree totally with pulling his weight in the home and proactively - he should not wait for you to ask him to put a wash on or make dinner. He should see that it needs doing, and do it.
More of a relationship one - what's his absolute worst habit/worst quality? (Ignore what it actually is, how do you feel about it?) Is it "It's really really bad but it only happens occasionally, so I can live with it" - you can't. Is it "I wish he wouldn't do that but probably he'll stop as he gets older" - he won't. Is it "Some people would find it a deal breaker but it (genuinely) doesn't bother me" - check yourself that it really doesn't, but this is fine. You need his worst quality to be something minor and annoying, not worrying, not scary, not dangerous, not even frustrating. Do not expect him to change automatically when kids come along. Most people find that very hard to do and he might not even be assuming he will have to.
How do you work together on projects e.g. planning a holiday, planning a wedding, or another family orientated event, redecorating a room. There should be a balance. You don't want him taking over, you don't want him sitting back letting you do everything so that he basically just shows up. Can you discuss and accommodate both of your preferences, do you value his input because it's genuinely good, does he value yours? Is it a good outcome at the end?
What is he like as a person? Is he somebody that you want your future child/ren to look up to? Would you be proud if your child grew up to be like him, as he is now? Don't take on a project because you want to mother somebody. You want kids, you'll have them to mother. Choose a finished article for your co-parenting partner. See also: Shared or at least compatible beliefs about things like religion, diet (vegan/sugar/organic/alcohol etc), pets, discipline, cleanliness, socialising, money etc.
If he already has children, what's his relationship like with them/the ex? This is more complicated but essentially you're looking for signs that if you split up, he'd still have the children's best interests at heart, and not use them to hurt you. Break ups happen - people grow apart. You need somebody who would be pragmatic and fair in that instance.
How do you disagree? What happens? Are you able to discuss productively with both seeing the other's side? Or do you avoid discussions, does he turn nasty, do you end up feeling it's your fault, are you scared to disagree in case he leaves? It doesn't need to be perfect, but it does need to be safe (no threats, no violence, even directed at objects) and productive (there is a mutually acceptable resolution at the end, not just ignoring the issue or avoiding it).