A car with a broken axle will exhibit one or more behaviors, including making a sputtering or clunking noise when its gears are shifted or rumbling and vibrating when the car is accelerated or turning. Note that these are also the symptoms of a car whose axle is merely worn down, and so can be used as warning signs that it is time to replace the axle.
The car's axle health can only properly be determined by a certified mechanic, and the car should be taken to one immediately if it exhibits one or more of the above symptoms. This is because a car with a broken axle cannot safely be driven. After it has broken at the universal joint, the car will only be capable of revving up but otherwise incapable of movement.
The axle can be damaged or broken by being overloaded or by having a bad carrier bearing or by the car encountering a bad bump in the road. A damaged or broken axle may also lead to further damage to the car or injury to the driver and passengers. As with most car issues, the damaged or broken axle should be only be examined by a certified mechanic in order to ensure that no further damage is caused to the car in the course of fixing it.