To test it, you'd need to get the amp to a temperature that you know should have the fan running and measure the voltage on the terminals of the fan. If the fan is a 12v fan and has 7v or more, it's likely that it should be running. If it has anything near 12v and isn't running, it's defective.
Also check to see if the blades turn freely by spinning it (no power applied). As it spins down, it should oscillate a couple of times at the end of the last rotation. If it doesn't, it's likely that the lubricants in the fan have hardened.