I have a friend, and she claims she is a sang vamp. I really don't know if she is telling the truth or not. How can you tell if someone is telling the truth about that without watching them drink?

Other than that, it is hard to tell in ways.
Other than more obvious physical things, like paleness/bad resistance to sunlight, I've noticed that our energy levels are very different. They drop quickly when we haven't fed, and after feeding we just bloom with energy, unless we have to settle for energy we could get psychically (but many do not like that and prefer to stick to blood, you should ask her if she considers herself 'hybrid' or a cross between the two.) When feeding not on blood, the energy levels go up and down in strange ways.
Take some time when you are talking to her and study her throughout the week- does she match up with the physical attributes at all? Does it seem like she is drained when you ask her if she has fed recently, and she replies that she hasn't? If so, she could be, but the best way to know for sure is to see her drink. Even then, she could be just a fetishist. It is hard to tell if you haven't been around the energies and studied a true sang.