I flew an Mi-8 for a number of years and for whatever reason there was no turn and slip indicator on the panel for the first year. I put a string on the wind screen and used my proprioceptive feel ( my butt) to sort of estimate where the zero side force was using anti-torque pedals in forward flight. We had a slip indicator installed later and the agreement between the string and the slip indicator was just sort of okay. Not very scientific but better than nothing, I guess.