@bumpitybumper thank you for your reply, which was lovely, and I think you're right, we are quite close in our views on this. As you could probably tell this subject tends to trigger a lot of anxiety in me about being believed 
I guess I was trying to emphasise that abusers, who are miles away from decent but imperfect parents, will hide behind the "not perfect" label and that is why I'm eager to make a big distinction between abusive and not perfect. But you're right, there is a bit of a spectrum, I mean I've seen that in my own family. There's a spectrum of intent, from they tried and failed to be good, through they didn't prioritize their kids through to they actually derive pleasure from seeing their kids upset. And there's a spectrum of effect as well, depending on which parent it was, how the other parent responded, and what other allies the kid has, family or teachers etc. But I worry every time I see a thread like this that someone who's experienced abuse is going to see this and think, yeah, that's me, it's a matter of perception, my mother/father/whoever is actually really well intentioned, I'm just taking it the wrong way. And then it's a long long detour into family therapy and non violent communication and all these sorts of things that would work fantastically well with people who are nice but misguided, but will just give abusers yet more ammunition against them. When what they actually need is to recognise the abuse for what it is.
It's not easy to recognise as we've both acknowledged. I think you know it if you've seen it, you develop a bit of a nose for it but it's hard to define. One way to know I would say for sure though is to see how they react if you enforce a boundary with them. Something small, like asking them to come to something on time if they're usually late, or telling them no those spoons don't go in that drawer. If they basically take it on the chin, they're a good egg. If it sparks off any gaslighting, the cold shoulder, tons of justifications or if they do as you ask and then find some other way to punish you, then you're dealing with someone who had the potential to make your life extremely unhappy.
Have you read Lundy Bancroft's book about abusers? I can't like I'm on mobile, but he profiles abusers and their methods and it's very good and although it is about abuse of women by men, it works for any relationship including parent/child.
Fishcanfly I've always thought the reason you heard more on here about toxic mothers and MILs is to do with what opportunities exist for toxic people at different times in their lives. My toxic mother was nice enough to my granny - she had no need to try and torment an elderly woman who she didn't live with, she had me and my dad at home to feck about with. Once I moved out she doubled down on my dad and when he died it was only then that she started giving us grief again. Your range of victims decreases as you get older.
I'm also, I have to say this again, it's a weird argument to me that if I accept that my mother is toxic, I should worry more that my kids might not want to know me when they are older. Surely they're more at risk of ditching me if I expose them to her? Which would be an act of abuse, to have her in contact with them, that's what my therapist said, and I agree with her.