How exactly could a man tell if a woman has a high N-count?

So with all the high N-count questions, lets talk about something more practical.

How would a man, assuming one who doesn't want a relationship with a high N-count woman , how would he tell if a woman in fact does have a high N-count?

For the purposes of the question, we are going to set "high" at over 17 partners. I am aware this is a subjective matter, but if were calling anything more than 2 high it's not a good discussion.

So, how could a man tell? In my experience it's easy to gauge whether or not a girl has casual sex or meets guys at parties from FaceBook photos, or if she has 50+ partners (from back talking to my bros), but beyond that the 8-20 range is hard to guess.

Thoughts?

Bonus: If this girl said she had an N-count of 3 would you believe this is an accurate answer?