You know you are beautiful when you catch guys looking at you. It is also an attitude to be cute because no guy will notice a hot girl who hides her face and is shy all the time. Physically, beauty is to take what you have naturally and enhance it the best you can with some natural-looking makeup. It's an art form and with many years of practice, I now have such good products an technique that people are surprised that I wear any makeup other than eyeshadow which is meant to be noticable. Hope this helps!
First off, I didn't have a boyfriend until I was 20 and in the end it matters what you think of yourself. If you think you are pretty, cute, beautiful and have that kind of confidence you will present yourself such as that. If your self-conscious with the way you look, and are negative about it, you will get negative reactions. Just because you haven't had a boyfriend doesn't mean you're not beautiful.
You know you're beautiful when YOU really believe it and have that confidence. If the only way you believe you're pretty is when other people tell you that, then you'll never be genuinely sure of yourself.