She's a good kisser
It takes all kinds. If you like the way she kisses you, then she's good for you.
She's a good dancer
Even if you're not, the way she moves her body vertically may be an indicator of her horizontal skills.
She could be a yoga instructor, a ballerina, a contortionist... basically, these allow for more possibilities in bed.
She gives seductive looks
Maybe she cocks an eyebrow, or offers other flirtatious or coy looks. Hell, maybe she's undressing you with her eyes. A woman's regard is the most important indicator -- of her intentions, her anticipation, and her confidence, which is directly related to her skill. Both the amount of eye contact she offers and the type are important.
She enjoys tasting her food
You won't always get a chance to see a woman work over an ice cream cone or provide some other obvious oral showcase. Still, whenever things are going in her mouth, she has an opportunity to employ the oldest of sexual metaphors. What does she do with this situation? Does she handle/cut up the food daintily? Does she put it in her mouth sensually? Animalistically? Absent-mindedly? Does she chew like a lady, or like a dog? Does she even seem aware of the messages she's conveying, like it or not? If not, she may not be interested in you in a sexual way.
She talks about sex
She often makes sexual references and discusses what she likes/does in bed. If she does it too much, however, she may be overcompensating, and could just be a tease. Does she seem to know what she's talking about? The important thing is that she be open to talking about it, or better yet (because it's more subtle), that sexual innuendo not put her off.
She doesn't talk too much
Loosely speaking, I've noticed a bit of an inverse correlation between chattiness and bedroom skills. That is, they might be more used to impressing with their words than their actions, or may even be consciously trying to compensate for one with the other. Quieter women, on the other hand, are often demons in the sack. They prefer to express themselves non-verbally... although they get vocal when you get them in the right spot. Put another way, if the woman won't be quiet, do you want to deal with that during foreplay, or especially after run-of-the-mill sex?