You could always TASTE it and see if it has a familiar flavor.
But if that's out of the question, you could try shining a black light ($20) on the underwear and see if the stain glows. Semen will glow but vaginal discharge will not unless a dye has been added to it AND she has a yeast infection. But that still wouldn't be conclusive without a sperm test ($50) or a dna test ($500).
As for the hair, if its perfectly straight, than it could be hers but from a different part of her body. Use a loupe and you will be able to tell no matter how short it is. But if it's thick and curly, then it's probably someone else's pube.
You should also realize that if you're sniffing around her used underwear, then you probably have other reasons for your suspicions (I do most of my own laundry and I never notice any of my own stains). So you probably have a variety of OTHER things you can check up on (phone logs, credit card charges etc. ) to see if she's cheating, or at least question her on.