In case you haven’t seen it yet, Kate Beaton has a wonderful Q&A about becoming a webcomics creator up on her Tumblr:
You can’t ask “how can I tell if my comic is good,” because you can’t think about it like that. You HAVE to know that your comic is good, that it has the potential to be good if it isn’t now, that it has good qualities, that you are not just throwing ideas onto paper and wondering if people will like it because you have no idea. You have to know what’s good about it, or you won’t know when it’s bad, and you’ll never know how to make it better. You don’t want to be one of those people who doesn’t have the skills to know whether what they make is good or not. Self editing is simply something people learn over time as they strive to make their comics better, people with the skill to know the difference. You need that skill, so don’t be shy about it, look at your comic and think to yourself about what you like and don’t like, and be honest about it. Then, like everything else, just keep working.
Now listen – even people with the most talent and skill in the world will suck super badly when they start to make comics. If your comic is bad right now it doesn’t mean that you are bad at making comics. My first comics were awful. Everyone’s are. Some people are awful for a long time then stick with it and get good. But you have to know you can.
If that last “(You can.)” doesn’t win you over, I give up. There’s lots of good stuff in the Q&A, and it’s as charming as all get-out; go read.