how can you tell if a fuse is bad

When you pull the fuse out, the center of the fuse looks like and upside down U. Is the U is broken it is burned out.

The hard way: pull it out and hold it up to the light and see if the filament is still intact. The easy way: buy a fuse tester, press the two probes of the tester against the two spots on the top of the fuse, if it's good it will light up. Saves you the trouble of pulling the fuse out. I recommend getting a small pack of miniATM fuses from a parts store and just keeping them in your car, it will have a tester in it, and you never know when you'll need a fuse. - BTW, if all your fuses check out, and you still don't have brake lights, you may also want to check the brake pedal position switch.

Sometimes a fuse will look good and it is not. This doesn't happen very often. An ohm meter works well for testing fuses out of the vehicle.