It might be better to wire your switches Normally Closed so that if a wire breaks or comes loose, it trips the limit switch input in Mach4, but I suppose that's note strictly necessary! It IS nice you have them wired to individual inputs, unlike in series like lots of folks do to save inputs!! Wired that way you can see which one trips or which one is disconnected or has a broken wire right on the diagnostics tab.
Should be able to go to diagnostics, if the led for a switch is lit when you go there, and assuming your not SITTING on the switch, you're configured backwards, reverse active high/low. If it lights the led when you trip the switch by hand, it's good.