Yes, people can tell if you really mean the compliment or your just saying it out of politeness or because you think you should. When you really mean it, they'll get the feeling that you're taking a genuine interest in them.

Try it out on some women you feel you want to say it to and see what reaction you get. Don't try to figure out in advance how to say it - just say it. The confidence will come later after the practice. That's the way it always works.