What do you do if you are unsure if the other person feels the same way as you do about that person?

Throughout my life I had hesitation telling someone how I felt which has been a good thing since I didn't really liked them. Some girls have considered me a friend and nothing more, but recently I felt about someone more than a friend. Basically I like her but the same reason bugs me, I don't know if she feels the same way. I have even tried to send some messages like saying she looks beautiful on a photo on FB, but that isn't enough to get the message other than directly. I have gotten advice like, just go for it, but like that quote, "it's easier said than done". I don't know what to do, and at this rate I'll end up alone. What do I do? If anyone has had this same situation, your help is most appreciative.