The very definition of 'beautiful' is "pleasing the senses or mind aesthetically."
In other words, there basically is no way for an individual to know, because whether they are beautiful or not is completely dependent on whether other people find you visually appealing or not.
You kinda just have to take people's word for it when they compliment you. And likewise, if people don't compliment you, you have to realize that this is just the subjective opinion of others, and it doesn't mean that you're not beautiful, because odds are that there is indeed someone who does find you beautiful. And guess what? Even the most beautiful celebrities still have a bunch of haters constantly telling them that they're ugly. So that's really what it comes down to, is learning to take compliments when people tell you that you're pretty, and let the haters roll off of you because it's just their subjective opinion. (And again, being beautiful completely depends on whether others find you appealing, so beauty in and of itself completely depends on the personality of the person looking at you. There's a lot of people that you shouldn't care whether they think you're pretty or not because they have a distorted narrow view of what being beautiful is in the first place.)
Me personally, I'm of the school of thought that being female in and of itself makes you beautiful. There's so many of you who post about how they don't feel beautiful, and I just want to jump through the computer and give you a big hug and tell you that you are beautiful.

(Plus, as was mentioned, if you find yourself appealing in your own eyes, you can call yourself beautiful, because again, being beautiful just means pleasing to the senses or mind aesthetically. So if you find yourself pretty, you can call yourself pretty with no shame, because you fit the definition.)