I live with someone who is disabled, but "doesn't look like it." So I know what you mean. It's impossible to explain, really. And if you try, people want to jump on one word or phrase and think they understand. We have stopped explaining.

Queer is a good word, because it's original meaning is "strange or odd from a conventional viewpoint; unusually different; singular." It's a good word to use, because it is such an umbrella term.

I always think of the old saying, "All the world's crazy, except me and thee, and thee's a little bit queer."