MOT_SPIN_MAX could solve your problem.
https://ardupilot.org/copter/docs/parameters.html#mot-spin-max-motor-spin-maximum
Regarding OneShot125 output monitoring, there is no more easy way to visualise it unless using oscilloscope on Rc out motor channel. With 3.5.x or 3.6.x arducopter, RC_OUT log data was in the 125-250 range when Oneshot125 was activated. Now, the RC-OUT (log data) is in the 1000-2000 range.