There’s not much you can know about sexual chemistry without having sex. You can’t judge a girl’s sexual prowess by how she talks or flirts.
But if there’s any somewhat-reliable indication, it’s good, old-fashioned kissing, which is way more important to sex than it’s given credit for. If you feel she’s a bad kisser—she’s tightlipped, or slobbery, or bangs teeth, or doesn’t know how to use her tongue—that could very well translate to how it’ll be in bed.
Having said that, there’s a lot that can go wrong in the sack the first time, when you’re both nervous. As you get more comfortable with each other, the true chemistry will begin to emerge. So give it a few tries before you give up completely.