I only judge the parents that are very clearly doing a shit job of it. Like my partners friend, he has 4 kids, doesn't parent any of them, not sure he pays for them and yet thought he was father of the year when he took one in for a while. 
Or a family friend who has kids, they were unruly as young children, going round hitting people, talking back at adults in a really horrible way etc and she would stand there and just laugh, did nothing about it. She now wonders why she has zero control over them now they are getting older, constantly in trouble at school etc. God knows what they will be like as teenagers. She could do something, anything, she just can't be bothered. Nor can the dad, he is completely uninterested. The kids have no chance. They are looked after well, no abuse or anything, but they aren't disciplined at all.
It's those parents I can't stand. I don't tell them how to parent because what's the point, they won't listen, but I do judge them as they are awful. I get it's hard, but do something, standing back and laughing is just teaching the kids that the behaviour is fine and it's not.
Being a parent is bloody hard and you never know if what you're doing is right, but generally speaking, if you are there for your child, take care of them, feed them well (or as well enough as they will eat, some are fussy, I was), and you're trying to teach them right from wrong etc then you're doing a good job. Kids have tantrums, they all do, I feel sorry for a parent who is dealing with a tantrumming child in public. It's doubtful they are like that all the time, just tired or hungry etc.