the sensor for feed back is sometimes usefull, but actually we can change this kind of sensor using physical limit (usually in our mechanical design).
we able to design the working area of our stepping motor, suppose just 1 rotation (so we can't do more than 360 deg movement) or we can make 10 rotation for working range, and we can't exceed this.
during our initialisation, for the first time on, then we can forced the stepping motor move to the zero point (we can choose max or min point in the physical limitation). we able to do this using timer (we must calculate it, it depend on the working area of stepping motor), then after timer is complete we can assume that the stepping motor already at zero point. then we can move it according to our need and saved the last point position.
this kind of design is not robust (not good), it's possible that during the initialisation there is some disturbance, but this system is very simple, no sensor needed.