You gawk a lot at people and develop an eye for what attractiveness means. Don't ask people, that's almost always useless, unless you happen to run into an expert on this. See what your eye responds positively to. Then evaluating yourself is as easy as keeping a reference feature in your mind up for comparison when you look at yourself. Keep in mind that attractive people are not all identical; there are attractive and unattractive versions and combinations of any trait.

There are also some things you could do to get an eye-opening perspective of yourself – ever looked at yourself through a second mirror forming an acute angle to the first mirror, so you can see yourself from the side view? I guarantee that the first time you do it you'll feel very surprised. Same thing when you're filmed talking and then watch the footage. Images that are flipped horizontally relative to your mirror image also help you notice asymmetries. The point is that the eye notices a lot more when the image is even slightly unfamiliar.