Well... agreed. Like I've said, 99+% of the time, that's the reliable barometer. In some fringes, it's not.
Last summer, I saw someone in the park where I walk my dog. Young person, 20 or so, and the most perfectly androgynous human I've ever seen. I wasn't eight feet away, in perfect lighting, and couldn't figure out if they were male or female. It was unnerving because I couldn't clock them as anything.
With most people I clock what they want to be seen as, which usually jibes with their sex. Some guys are feminine, some gals as butch, but something just feels right about the ID. I might see a man who is unquestionably a male, but if he has long hair and makeup and a dress on, I'll probably take the shot and treat them as a woman unless I'm told otherwise. I just don't feel the need to argue with them that it's 'biological fact' that they are male. In our social interaction with each other, I don't find that to be a hill to die on