OR you judge by other people's reactions to you. If girls are always staring at you when you go out, then that's a normally a good indication that you are somewhat good looking. The good thing is that girls' opinions vary, so there's lots of different versions of what is good looking.
I don't know if this is the same for guys, but I guess I'll go ahead and share why I believe that I am good looking (not to sound conceited). I've been told on various occasions that I'm pretty/gorgeous/cute/beautiful by adults, such as my parents friends. I notice guys looking at me when I go shopping at the mall and such. I've heard guys make comments about me as I've walked past them. Some guys have straight up told me that I look good. No one understands why I've never had a boyfriend, because they all think that because I look good I should of had one before.
Sorry, I didn't mean to make that so long. Anyway, when you see a girl that you think looks good, what do you do? Try to turn that situation around.
Assume that you're good looking and don't take rejection personally. Every guy gets rejected and guys who say they don't are not telling you the truth. To use a baseball analogy, the best major league hitters don't bat a thousand. Barry Bonds has a career average 0.298, which means on average for every 3 time he steps up to the plate he earns a run 1 time.
I could go on with the baseball analogies, but the point is you have to be willing to step up to the plate if you expect to get a hit. Furthermore, you may get a hit, but the hit will likely just get you to first base. Most of the time, the inning will be over before you ever get to home plate. If you're lucky, you may hit a home run every once in a while, but it takes time to become a major league slugger!