It's a good question.
But in the same way that you can accept that I'm heterosexual by an internal feely thingy that I just feel and know, why can't you accept that I can feel reasonably certain that someone isn't dissembling about their own sexuality, using the same kind of mechanism?
Or, if you think you can justifiably call my judgement into doubt (and you, of course, can) why not apply it to my judgement of own sexuality too?
I agree that my judgement of my own sexuality is likely to be more accurate (assuming I'm being honest with myself - though there's no reason to suppose I always am), than my judgement of someone else's sexuality. But there's probably a margin of error in both cases.
In the end, I suppose, it comes down to how well you know another person (and yourself). If someone acts, and talks, consistently on a certain issue and gives no indication of dissembling on other issues either, and they do so over a period of time, I don't see why someone can't be averagely confident about them. Even though, or especially because, it's no big deal one way or another. I can understand someone dissembling if there's a reason to do, but to do so with no reason? Is that likely?