There are many airplanes that have no artificial horizons and are flown in mountainous areas.
Therefore we must go back to basic training to determine how one maintains a constant angle of bank and altitude turn in that environment.
I found Chuck's question an interesting one, because I can't work out the answer.
I've instructed turning on gliders, but only in the flatlands. There, the horizon is pretty uniform.
I've flown in hilly areas, and there I fly the turn by feel, with regular checking on the ASI to ensure that I've not been fooled by the horizon playing fairground tricks on me. The closer to the hill, the more I check.
I'm not sure how to translate this into teaching - maybe in a powered a/c with a shallow angle of bank, monitoring the ASI would do the trick? But in a glider at, say, 30 degrees of bank @ 50 kt turning 360 degrees twice per minute, I'm not sure that would do it for a student. No A/H, and although the yaw string is better than the ball for slip it tells you nothing about angle of bank or attitude.
My
guess would be to teach the turn, prompting as required, until the student gets the feel for steady angle of bank and airspeed (probably judged by airflow noise in a glider), and then teach monitoring the ASI for checking purposes.
Chuck's answer would be interesting.
[BTW, I don't currently instruct, and only used to instruct at the introductory flight level, in case anyone takes this post as an indictment of UK glider instructor training.]