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.
Ok the fan is working again BUT of course another problem has come up. When I play the stereo the subs cut out if I play the volume over 15 I normally play it at 30! I think it is in the amp settings I dont know whats what on them can you help;
If the amp only shuts down when it's driven to relatively high volume, you need to confirm that you have sufficient power supply voltage. With your multimeter set to DC volts, the black meter probe on the ground terminal of the amp (not on the point where the ground wire connected to the vehicle) and the head unit on (so the amp will have remote voltage applied), touch the red probe alternately to the B+ and remote terminals of the amp. If the voltage is below ~11 volts on either the B+ or remote line, you need to check the wiring feeding whichever line is too low. If 'both' the B+ and remote turn-on lines are low and your battery is fully charged, you may have a bad ground connection.
Thank you so much for taking the time to help me. I am obviously very new to the whole wild world of maintaining my car stereo. All of what you said was totally confusing to me BUT I am resourceful and navigate through the internet well so I will interpret your suggestion I just wanted to say thank you I do appreciate your help.