Big subject. Best writeup I've seen is Rory Miller's "Meditations on Violence" (though it's not perfect by any means.) Basically, there are many types of violence and the question is: what type are you preparing for and why. You could write several books on that subject.
what makes someone a professional martial artist teacher? How do you tell compared to someone whose just sat on youtube too much. And should i be worried?
Depends what you're preparing for and why. In your case: just getting exercise? Not a big deal. Qualify for good performance in a particular tournament? Depends on the organizers, the ruleset, the "meta" strategy of fighting techniques used, etc. Be able to defend yourself in any situation? Almost impossible.
BTW I wouldn't carry weapons in a public park, even if I was just training, but your local laws and policies may vary.