You "know" the position of a stepper motor by keeping track of what step you have moved it to previously. Unless there is a massive fault load the stepper motor will not "lose steps".
If you also need to know the starting position you can use a single sensor like an optical or hall effect sensor. In some applications they just juse a mechanical endstop, and on startup just move the stepper motor as far as it can go (which hits the endstop) then after that its position is known.