When A Man Sleeps With A Woman, Can He Tell If She Is A Virgin Or Not?

There is no reliable way to know.

Some women have intact hymens, but many virgin women don’t, either because they were born with an incomplete hymen (most commonly, the hymen doesn’t completely cover the vaginal opening), or it has torn via tampon use or naturally by the pulling as the adjacent tissue (corona) stretches further apart. It’s also usually only a fairly thin piece of tissue, like skin.

All of these are normal variations of the hymen that women can be born with:

So many women don’t bleed, or don’t bleed enough to notice, the first time that they have penetrative intercourse.

The only reliable change that happens when a woman has sex for the first time, is that she gains the experience of having sex

There is no 100% guaranteed method to assure that a woman is a virgin. 

Not only can hymens be 'repaired,' but they can be created anew if a woman doesn't happen to have one -- as very many women don't.

Further, even the blood that ostensibly demonstrates virginity can be faked with easy-to-use capsules that can be bought over the Internet.

Probably the most reliable source for the information is for it to come from the woman herself as long as you trust her.