I go for the audible and visual indicator..........................grinding noise and sparks
Okay, perhaps not

visual for me, and my rear pads had an annoying squeal when they were getting low, a feature built in and still had plenty of material left.
I use OE pads on mine, less brake dust and superb performance and they are Mintex. My rears were replaced before christmas and a shade under 60k. The fronts are still okay for a few k yet, but I have spares.