I'm not aware of a general system variable available in the Fanuc package that will allow you to achieve what you want, but some MTBs supply an enhanced Macro executable that has load monitoring, Nakamura being one. If I can find the detail I'll post it. You never now, it may be a variable that is available. but just not listed by Fanuc.
Fanuc has iLimitS CNC Software Option for Machine Process, monitoring load, speed and coolant monitoring with Macro
Regards,
Bill