One surefire way to know is if he brings up other women to you casually and talks to you about issues or problems he's having with them. Any guy who's trying to get in your skivvies is DEFINITELY not going to be talking to you about other women. He's going to give you the impression that you are the only person he's dealing with.
Another way that you can tell if you've been friendzoned is the level of flirting or playfulness he exhibits around you. Any guy that's interested in you will be ALL about the innuendo. When we want to get you in the sack, we make it a point to provide you all the opportunity to say, "hey, do you want to get in the bed and make rolling pyramids for a three minutes?" While I realize that's more about sex than anything, a guy who likes you, will want to sleep with you. If he doesn't, he definitely just views you as a friend.
In fact, that's probably the best way to know: if the guy that you're interested in is showing no sort of sexual interest in you - touching, flirting, giving you "the eyes of desire" (so lame), coaxing you into the Egyptian cotton - it's quite likely he views you as a sister and you can spend the night with him in his bed and you'll both sleep better than two babies strung out on sleeping pills.
Oh yeah, he might actually say something like, "you're like a sister to me" or "you're the best, that's why I love you like a fat kid loves cake, but he didn't really want the cake and would rather have ice cream, but the cake is fun as hell!"
Bottom line is, with guys, it's pretty easy to tell. Either we want to hump you or we want to play innocent board games where there's no chance in hell you can end up naked with you, like Parcheesi. I'm guessing NOBODY ever got randy playing Parcheesi.
It was written.