The signs that let you know if your fans are bad include unusual noises, rattling noises, humming noises, fan blades turning slow or not running at all.
If the fan is making squeaking noises, turning slow, rattling noises or running at inconsistent speeds it's a sign that the fan motor is likely bad and is gonna fail soon.
If the fans are cooling fans for a computer or a car the computer or car engine may overheat when the fan is not operating properly.
If it's a car that the fans are on the check engine light may also come on and the temperature gauge will go up.
When fan motors go bad they often go bad due to the bearings wearing out which could cause the shaft of the fan motor to get loose and start rattling or it may just simply start slowing down and seize up.
Dust on fan blades can also be sucked into the motor and cause wear and tear on the fan motors shaft and bearings and cause it to seize up as well.
Any grinding, rattling, clicking or excessive humming noises from a fan motor or fan are a common indicator that the bearings are worn in the fan motor or there are also loose parts.