The rear brakes on my GT have two ways of letting you know when it is time for replacement, IN THEORY!
The first, is an electronic sensor that when the pad friction material is worn to its limit is worn through and breaks a circuit which tells the onboard computer and turns on a warning light and related symbol on the dash board.
The second theoretical way to check for wear limits is holes in the pads that align with holes in the caliper. This SHOULD wear though before all the friction material is worn off the pad so you can visually check if the pads are servicable.
Another theory disproved!
I felt that the pads were worn out, but it is hard to see the pads while the caliper is assembled on the bike, so I pulled it off and gave a quick look.