How can you tell?

How can you tell if someone is qualified to be a parent? I mean, you have to take classes and past a test to get a driver's license. You have to pass finals each year in school just to move up a grade. You can't sell houses, join the military, or even serve food without taking classes and/or passing a test. But, no, it's perfectly ok for anyone to have a kid. You just need to go have sex and get pregnant. Now, I know that this may seem to come out of nowhere, but after a week of taking care of another person's child because that person is too into drugs and their own concerns to do it, I get pissed. And, since this child is older, the bad behaviors have been there for years. Now, is this the fault of the child, who has known no other lifestyle, or is it the fault of the parent, who made the decision to have 3 children, and then ignored all but their most basic needs (and even then, not always) so that they could go out with their friends and get high non stop. Yeah, I chose the parents as the guilty party, too.