I can only speak for the US, but a lot of airports have those full body image scanners, and they may pull you aside no matter how well you are tucked. They are supposed to tell you that they have to search a certain area, and they offer to do that behind a privacy screen as well. Further, they should have a TSA agent matching your apparent gender perform the search, and they are supposed to be transgender aware (meaning they should be respectful of discrepancies between appearance and gender marker).
That is a bunch of "shoulds" as in, that's what their guidelines say. You may run into the occasionally power-happy jerk.
You could always get a letter from a doctor/therapist stating that you are transgender and your appearance differs from the gender marker (or whatever applies).