I assume this was pre-9.0? Previous to 9.0, there was alway a "tell" on the IC (or, I'd assume the main screen for Model 3) if auto lane-change would or would not be allowed...if the lane you wanted to change into (left or right) was not explicitly shown (i.e. there was not another set of lane lines beside the one you were currently in), then that was the clue that auto lane-change wouldn't work.
So driving on a surface street, the IC would just show the current lane I was in and therefore I knew I could not auto change lanes on AutoPilot. And even some stretches of highways/freeways would (sometimes inexplicably) not allow the lane change. So if I was on the freeway and tried to auto lane change to the right and it didn't work, I'd look down at the IC and see that maybe it was only showing my lane and the lane to my left. That meant, at that very moment anyhow, that it would let me auto lane change to the left, but it would not allow me to auto lane change to the right. Of course, after a few seconds to a minute that situation might change.
Post 9.0, I find I can auto lane-change some surface roads that are not highway/freeways. Some can and some can't. Not sure if the "clue" above will apply to 9.0 as well, but it certainly might. If so, you can glance at the lanes being displayed before attempting to demo to your friends.