There are many different ways that you can tell a women that she is beautiful without sounding like you have any sort of desire for her. You can tell her that she is beautiful and that your glad to have a beautiful friend like her.
Compliment a particular attribute, like "you're eyes are a lovely shade of green" or "you always style your hair so nicely". Wouldn't hurt to preface it with "I hope this doesn't come across as a come-on"
It depends who this woman is to you....either tell her you just want to compliment her on her natural beauty or say she reminds you of some distant relative or friend that was/is quite lovely.....Id plainly tell her you are not flirting or don't desire her in that capacity, just expressing what you see.
I think it depends largely on your relation ship with the woman and how close you are to her. If you are her friend, you should be able to tell her she is beautiful with out her jumping to conclusions about your feeling for her. If you are concerned that she may do this, you can say something along the lines of 'in a strictly platonic way I think you're beautiful'. But I wouldn't worry too much about it, I think a man can call a woman beautiful with out her assuming he has feelings for her.