Sodium pentathol. That's your only option if you really want the truth, aside from torture. You could water board them both, but you wouldn't need to worry about either of them afterwards, haha. Honestly, I'd get a different chick man. Jealousy is one thing, boundaries are another. Think of it this way bro. If you were coming home from work every day to this guy hanging with your wife, would you be ok with that? He may be friend zoned, but that's not always concrete, and neither is him being a homo. Some dudes fake being gay just to be around girls they like. It wouldn't be bad, but then you would end up seeing them topless or trying panties on, and have to end up explaining why you were French kissing her vagina, haha.
Refs: Tito24, RiverSideRider, Stussy, Nettyspaghetti, Daniel24, Freddydapice, BrokenBMX, SuperstarDK, HardBMX_Tim. There's more, but I'm too lazy to look them up, haha.