haha, they just came out with the pad for thong wearers...that is funny to me bc i used to look for panty lines as a way of checking well beforehand.
Now, i can normally tell a change in teh way a girl smells, but this takes a while of getting to know them to tell when it changes. i'll normally confirm or ask if i don't know, then base the scent around that. also, a lot of women will start to wear a bit more perfume or will change what they normally wear.
if you're worried or wanna know, though, just ask. Most girls i've dated have been very open with their cycles and time of the month...I think it's bc they know i'll give more backrubs and massages around that time to try to keep them feeling good while they are bleeding...
oh, and one lesson learned from southpark..never trust something that bleeds for 7 days and doesn't die
j/k
but seriously, just ask, every girl is different..