Most of the girls I've met who have problems with their appearance realize they are beautiful, but haven't
accepted it yet. Whether it is for
personal reasons ( they don't know if they are
capable of acting out the
role of a beautiful woman in society) or whether their
definition of beauty may be slightly different from what they see in the
mirror, most women who are genuinely
attractive seem to know it at some level or other. Speaking from personal experience, telling a
girl that she is beautiful just when you
meet her ( depending of course on circumstances ) is a very
nice thing to do. It will at least get you a
smile, and if you say it as sincerely as you feel, probably a
blush as well.
That being said, it does make the girl very concious of herself, and perhaps start to question her own beauty later. Which has been mentioned above. This is bad, obviously, and it is wise in those cases to hang around, change the subject, and talk of other things. Its a good way to get to know her anyway.
Friendships, and girlfriends are a bit different. The person you are complimenting is obviously more than a two dimensional image on the street, on in the bar. For this reason if you mention beauty often as something you admire in them, it will make them incredibly self-concious, and also wonder why, in light of all their other qualities, you only fix on their beauty? This is something women do wonder about occasionally and guys as far as I can tell, don't. A genuine compliment, repeated from the heart, once every few days, can seem very shallow, and fixated to a woman if it becomes predictable. It makes her feel painted into a corner, boxed in. Caged...
I speak of course from the personal experience of losing several girlfriends this way. Time, and wounded hearts, lead to sophistication, and also to better climes. I learned if not to make the same mistake twice, at least to know when I was making a mistake and stop, and backtrack. Then later not to make it at all. Which was another mistake, because if you don't mention beauty at all, a girl starts wondering what the matter is with her. Then after a few more years, I realised that you only bring beauty up when she draws attention to it, compliment her, when she dresses up, changes hairstyles, or even when the light changes considerably. women love to be adored. At least that's what I think. Beauty is a part of that, we shouldn't make her feel ashamed of her beauty, but accept it as part of herself. It may not last long, so enjoy it. Both of you, while it lasts.