I think attitude and the chest are the biggest parts of it, personally. I'm incredibly lucky to have been born with an unusually deep voice, so I was mistaken as a guy before I ever started trying to pass. If you're in a situation like I am, having a big chest and no having a binder yet, wardrobe manipulation is the key. Seriously. Jackets help, first off. What I've been doing lately is to wear two sports bras, a button up over a t-shirt, and then either my leather jacket or a trenchcoat. Granted it's winter and that's impossible to do in the summer, but it's worked pretty well thus far.
I don't think the voice is ALWAYS the giveaway, though, I've known quite a few bioguys with high voices. Keep in mind too, that people will always have their own ideas on what man is. Some people are so close-minded that they think any man without a crewcut and 6-pack isn't a 'real man'.
Sorry if that post was mostly useless, just giving my meager input.