I was scratching my head the first time I read this question a few months ago.
Since that time, I have read the same question from hundreds of readers! I think
a new urban legend/old wives' tale must be in the works.
No, you cannot tell whether a woman -- or a man, for that matter -- has had sex
by the size, shape, or color of her or his nipples. I am making a guess here,
but I think the origin of this myth may come from the fact that women who have
been pregnant do sometimes have larger, darker nipples. But this has nothing
to do with simply being sexually active.
Folks, I have heard hundreds of people insist that you can tell if a woman is
a virgin by everything from how she walks to something about her eyes. The
reality is that the only way to know if someone has ever had sex is to ask them.
Nothing magical happens to a person's body after they have sex for the first
time. Do people look different after they've ridden a horse for the first time
or had their first drink? Well, yes, but only for a few hours. :)
So, stop looking for ways to "know." Whether or not someone is a "virgin"
is such an ambiguous question as to almost be unanswerable anyway.