When do my brake pads need replacement? I have a bike that is almost two years old. It still has the original brakes (side pull caliper brakes), which seem to still be functional. However, they are worn and I am going to be doing a lot of biking out of town during the next two weeks. 

I think I know how to replace my brake pads, but I don't know if they still have plenty of life left or are about to go. What should I look for? How can I tell when they need to be replaced?
There must be some wear indicating lines on your brake pads. If you don't see them - it's time to replace the pads.

Here is an example of wear indicator lines: