My friend and I like the same guy?

My friend told me that she likes this guy, but I didn't tell her that I had feelings for him too. I have held back on flirting etc, as I don't want to get in her way or anything like that. But I can't help liking him. How do I know which one of us he likes? He keeps staring at me when we go for lunch and stuff like that, but he is always with my friend. Always. I asked him if he liked my friend and he said no, but is he lying? I don't want to seem petty at all, but how do I know if he likes either one of us? Thanks