Deus Ex does not have character levels. You receive skill points as you complete objectives, and then distribute those among your character's skills.

Deus Ex is skill based. Your "level" is irrelevant, not that you really have a level per se because nothing "levels up" when you reach an experience threshold. Rather, you receive points to improve your skills. So each individual skill level is what counts.

If you want to know how many times you've passed an experience threshold I'm sure there is a table somewhere. Try GameFAQs.

-EDIT: It just occurred to me that last line makes no sense. There is no experience threshold. You're just awarded a set amount of skill points for completing objectives which are in turn spent on skills.