Well, I asked my girlfriend last night about this whole sexy underwear thing, and she said that wearing sexy underwear makes a girl feel sexy. Even if the guy's never going to see a stitch of it, it'll make her feel more self-confident. So chances are, if a girl is out with you, and she wants to feel sexy and confident, than chances are pretty good that it's either a date, or she wants it to be one.

Of course she could just enjoy wearing nice underpants all the time, and you're just barking up the wrong tree, but let's just pretend that she wants a date. Of course, as rescdsk pointed out, by the time you see her underpants, you're pretty sure that she wanted a date, so I don't think there's any reliable way to use underpants quality as a sort of early warning indicator for a date.