How do I know if my boyfriend really thinks other girls are attractive or not?

So I've been with my boyfriend for 3 years now on and off... Mostly "on" lol.. I've always had a pet peeve for my boyfriends checking out other girls wether they are standing next to me or not (I guess what girl doesnt). We have been through a lot together, good and bad.. But i guess my wonder/problem is that he swears he doesn't check out other girls when i ask him if he does. I even say to him "com'on babe, your a guy. Every guy does." He replys by saying how of course he thought other people were attractive before he was with me but now that he has me he isn't worried about anything else. And he also says that not EVER guy is the same. Which i completely agree with, im sure there is a few guys in the world that actually dont check girls out wheb they have a girlfriend.. But i find it hard to believe i found that "2 in a million" ya know? So is it really possible that im just a really lucky girl that I found one of thoes 2 in a million? Sometimes i tell him id feel much better if hed just admit it to me that he does but he replys by saying that he's not going to lie and admit something thats not true and that im the only one for him and how perfect i am to him. I know this guy like I know the back of my hand and he's my everything and i know im his.. I actually almost want to believe him when he says that he doesn't look at other girls but then in the back of my mind i say "nah not possible he's a guy."... And regardless if he does or not... How can i let it not bother me so much? Like i mean it REALLY upsets me. And i know the most obv answer is i have something that the others dont have because he chose me, but yet is still bothers me. And like most women won't do I admit that I probably am a little overbearing about it sometimes but he usual keeps his cool and never gets frustrated with me about it.. he's really the best thing that ever happened to me and it just really shoves a knife through my heart to think he does this.

P. S. Thanks for reading this lengthy post!
How do I know if my boyfriend really thinks other girls are attractive or not?
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