Here's my technique for a quick judge of pad wear. Every time I swap tires I inspect the pads and compare them to a new set. Anything approaching 50 % on any pad and they all get tossed. I keep an old tooth brush in my tool box and give the pucks a quick scrub with some brake cleaner before slightly pushing them back in for more clearance for the new pads. This reminds me, I need to order another set of pads......