Remember in elementary school when you chased the pretty girl around the playground and grabbed her ponytail? She probably wondered why you were bullying her, having no idea that you were trying to let her know that you secretly liked her. Fortunately, you've both gotten a little better at giving and receiving body language cues, so pain is no longer required to get her attention. But a little touch is still a great way to let a lady know that you're into her.
The big rule in this is to make sure the touch is appropriate. Anything perceived as groping will likely land you a slap on the face and an escort out the door. But finding an excuse to grab her hand is a great way to test the waters and see if the feelings are reciprocal. Maybe it's legitimate, like trying to stay with her in a crowd, or maybe you have to make up a reason, like pretending to read her palm. The good news is, if she keeps her hand in yours, she probably likes you. But if she quickly pulls away, you may need to put on the brakes.