How do I rest z axis

[Doddy]

What could I use to monitor this? Any ideas?

Not what you want to hear - but almost certainly better to replace steppers with closed-loop.

You *could* attach rotary encoders onto each screw and - not sure of any COTS solution - knock up an encoder comparator to generate a fault signal. Rotary encoders are in the £10-£20 range for cheap, and an Arduino is the cheapest way to implement the comparator. But that's all a little bit of a bodge.