It's super easy to see when someone is exiting the roundabout, lights aren't necessary. When someone isn't exiting the roundabout the turn is much sharper. When they start to pull away from the center Island, that's the cue to know their exiting and it's your turn to go.

Also the outer lane should be exiting at the same time as the inner if not before. If you're blocking someone in, you failed to yield when entering or you're trying to go around further than the outer lane is supposed to (you shouldn't make a left from the outer lane, only right or straight).