Is he cheating on me?

I've been with the same guy since the beginning of January of this year (a little over 7 months). I started to get this wretched gut feeling about a month or two into our relationship. I've never been cheated on before but these are obvious signs to me. I was checking his Facebook account (he never gives me the password for it and all ways changes his e-mails password so I can't read them!) and I was asking a few girls how they knew them, One of them answered that they've been seeing eachother for a while. All though this girl is a lot younger than me (17yrs old) I don't know wether to believe her or not because of he immaturity and want to cause drama, I confronted him about it but he said that she is full of herself and just wants the drama, Don't you think that's a little strange? The other day he was sitting out in my car and I came back in from the store to get in to my car and there were make up smudges that I know for a fact were'nt mine... I have confronted him about this too and still he denies it to his very dying day, I don't know what to do anymore. I don't think he's ever going to tell me the truth?