The easiest way to check this is with a multimeter or continuity tester. If you have continuity from where you want to connect the wire through to the tool itself then it should work. I have the wire passing through the cable track up to the spindle head. I fastened a 1/4 blade-type lug to the spindle mounting bracket with matching connector on the wire so that I can remove it and hook up a clip if needed for whatever reason. Before I did this, I confirmed there is continuity from the bracket through the spindle to the tool mounted in the collet. Then when I need to probe a tool, I pretest (you can use the diagnostics page to check the LED, but I've added one right on the Program Run page) as a final check prior to use by touching the touch plate to the tool. I usually do a test probe using the Auto Tool Zero button or a manual G31 command from the MDI as well before running the program just to make sure all is in order.
Hope that helps,
Stephen "Highspeed" Kruse