Well, anime with high production values tend to be the best, even though it's still not guaranteed that it will be good at all (See production IG). So I usually just stick to whatever has good animation.
I do watch everything else for at least a few episodes. The way I tell if it's bad or not is by first looking for any meaning or direction. For example, a show I actually like, but isn't very good - Bamboo Blade - is pointless. It's just a simple story about a (girl's) kendo club. There's no real purpose, meaning, or theme behind that. Its likability is based purely on how you like the characters, setting, and premise.
Sometimes an anime can be more than that, though. Even without meaning, purpose, or thematic elements, a show can be enjoyed with direction and animation alone. That just takes a good sense of asthetics. If you don't already have that - school can help. (But going to school for anime is pointless, so just fuck it, and enjoy what you want)
I just looked over my post, and it's pretty shit, but I'm going to submit it anyway. There are plenty of things that make something in the animated medium "Good." These are just a few. (Also, I picked on Bamboo Blade because I like it. Didn't want to offend anyone)