Once you get some experience, watching height of your trailer becomes second nature.
I remember following our GPS in Durham, NC. Round a curve and see the low bridge. Instantly hooked a left turn. The only turn off before the bridge. Never saw any height limit sign. But I knew we weren't going to fit under that bridge. And I wasn't about to back up in traffic. Had no idea where the left turn was going to take us. But I thought we could figure in out. And we did.
The point is...after a while you just develop a "sense" for large towing. And there are a few things you just train yourself to look out for.