On a more practical level, unless you know the guy very well, you only have his word that he is str8. Believing him may very well be your right, but without any means of checking the truth, this statement has to be taken with a huge lump of salt.
Quite a few guys are being perceived as being str8 because of their lifestyle, not so much because of their sexual proclivities and the established pattern of their sexual behavior.
Unless we are talking about the situational homosexuality here, there is little chance that a genuinely str8 dude would really have any interest in messing around with another dude. Rightly so, str8 guys do not just define themselves as being solely attracted by the females but very much so as being opposed to having anything sexual with other males.
No doubt, a number of gay men are very keen on having sex with a guy they perceive as being str8. First and foremost, this is a huge self-validation, an ego boost if you will. "I am so irresistible that even str8 guys cannot resist my advances..."
The stigma associated with having sex with other men has largely subsided over the last few years. Hence, a few apparently str8 men perceive themselves as being open to the down on the low phenomenon. If no female is readily available, and there is this guy willing to help him get his rocks off, and no one will really know...
If it works for you, what's the harm in trying? If you have better means of getting where you want to be, by all means, take the path of lesser resistance, and save yourself the work...