whats going on?

my friend & i decided to take our friendship 1 step further, he has liked me for years but my feelings about him just started to change a few months back, we kept things quiet incase it was just a bit of flirting. we've gotten on really well & things has been good but when i ask him whats happening he says he likes being single, im pretty sure he's gotten rather friendly with another female friend, as i over heard 2 other friends of his saying they saw his txt msg & it said that she was waiting outside for him, normally i wouldn't of thought anything about it but if nothing was going on why keep quiet that she was giving him a lift? i like this other girl & wouldn't want to see her get hurt but i don't want to carry on in this way with him if he is seeing her even though i do like him. i've asked him b4 if he likes her more than just a friend & he said they're just friends. i don't want to sound like some jealous b itch but how do i find out whats happening without sounding like 1?