I have a good friend who is a trans woman. I've known her for about four years, and was pretty sure she was trans just from looks, voice etc., but the subject never came up because she never raised it and I certainly wasn't going to.
When I realized I was transgender and began making preparations to transition, I thought about ways I might raise the subject with her. I had decided long before that I wasn't going to come out unless someone was genuinely curious, because it's kind of a heavy thing to lay on a friend.
I started growing my nails long and gloss them, but she never brought it up and neither did I. It wasn't until I pierced my ears about six weeks before I was to go full time that she asked what it was about. When I came out to her, it turned out we had known a lot of the same people, gone to the same support group (not concurrently of course), etc.
Oddly though, there's been some distance between us since I've started full time, that I'm not sure I completely understand.