Normally it is the spindle load that is monitored, giving an indication of load current , some systems take it one step further and the feedrate is reduced when a spindle load is exceeded.
VFD's and most servo drives have an analogue output that is a direct ratio of drive current, this could be taken to an analogue input quite easily.
If you do not have analogue inputs, it gets trickier, although you could get a separate 3 digit digital display from Red Lion or other suppliers as a cheap fix.
Al.
CNC, Mechatronics Integration and Custom Machine Design
“Logic will get you from A to B. Imagination will take you everywhere.”
Albert E.