The title pretty much says it all. I think I need to get mine replaced, but I do not know how to tell.
More expensive brake pads will have a wear line. Replace these when you hit the line. Not so expensive ones should be replaced when the protrusions from the main block have been worn away.
Another decent way to tell is by how far you're rolled out your barrel adjuster. If it's out more than 5mm or so, it's probably time to replace the pad.
More importantly than any of these indicators, if you see any metal showing through the pad, it's time to replace.
Nice tips, +1. Although one can use the pad "to the end", it is usually not worth it, because the brake-pad alignment is much more difficult, you end up risking to damage the tire and even the rim (metal-to-metal contact), and, plainly speaking, the pad has already paid itself long before getting to this point. Mar 19, 2012 at 20:09
I would err on the side of caution. Rims cost a lot more than new brake pads. Kool-stop pads are under $20 and they are great pads.