You can ask your doctor. kellym may be right about the swabbing but they can also check by ultrasound. They can measure the amount of amniotic fluid by ultrasound to make sure you have what you should at this rate of your pregnancy. I have to say though, my underwear seem to just stay wet (damp) while I'm pregnant!!!
I think the only way to tell is to go to the dr. and they swab the area or panties with litmus paper. I think it will show up a certain color to indicate amiotic fluit.