unfortunately, you'll prob never know unless you meet an objective person who cannot lie. that being said, if you don't want to hear beauty is in the eye of the beholder (which i think its stupid too), what people think is 'beautiful' DOES differ, people will have certain tastes/preferences according to culture, time, race, fetishes etc.
the easiest way to objectively know i guess is to find some face symmetry app/website where you upload a pic of your face, and tell you how symmetrical it is etc.
i just googled it, and you can try the site they suggest : [link]
Also: Attraction of the opposite sex has nothing to do with 'looks'. Dunno if you've ever noticed, but couples tend to draw like a magnet towards people who posses similar outlooks on life, personalities and life goals or at least one of these combinations.
As you get older (not sure how old you are but i'm assuming you're still a teenager) the more you realize your interests and goals and work on these the more like minded people you'll meet. The more likely you are to meet guys who find you attractive when you're doing this, because you're in an environment where they'll not only recognise your outer beauty but they'll also see your 'inner' beauty too which is the thing the glue that binds couples together.
Be yourself. Learn to see the positive attributes you have and find physical features YOU like about yourself. When you wake up in the morning look at these physical features in the mirror and remind yourself you like these (until it sets in that you are attractive). Focus on all your positive traits and don't worry so much about finding a boyfriend. In-fact the less you focus on it the more guys will find you attractive because you're not actively looking to be with someone you're just being you and that's what guys like most more than anything else.
If you won't accept "eye of the beholder," would "different tastes" be satisfactory? I'll be with my mom or my sister and one will say, "That guy is really hot/handsome." And I will look and find them rather mediocre. Same with magazines voting on Best Looking Celebs; I might just not find them physically attractive.
I've never sought a romantic relationship (not yet, anyway), and for a long time I figured I would probably end up "forever alone." I didn't think myself very attractive. But guess what? I was asked out by a guy who did.
People will like what they like, and one man's trash is another man's treasure.
This is kinda a funny question. "Pretty" vs "ugly" is a very superficial thing that complys to societal norms, not what is the true versions of 'pretty' and 'ugly'. Like many of the commenters before me have said, the truth of the mater is that true beauty is in the personality, just as true ugliness is. We may see actors or people in our lives as the societally accepted form of a pretty outward appearance, but in reality, they may be the ugliest people of all due to vanity and self-centeredness. True beauty is found within. Just remember that.
You're not going to believe it... but no one is ugly. It sounds more like lack of confidence in yourself. I didn't have a serious boyfriend until my 20s, because I waited for the right person. Many of my friends did the same thing. Now I'm getting married. The rest of my friends who I used to envy for always having a boyfriend or girlfriend? They're still single!