Do a simple bench test.
Put the car up on a stand so it's off the ground.
Turn off the radio and car.
Try to turn the wheels with your hand.
They should turn with some resistance and you should hear the "whine" of the servo gears turning.
If it skips and jumps and/or you hear any "crunching" sound that is the stripped gears.
As someone else said you could also take the servo apart and see it but reassembly can be tricky.
If the gears are OK then move on to radio adjustments and so on.

^this^
To solve this issue on a more permanent status get a savox sc-1256tg or sc-1258tg servo!
They come with titanium gears and very fast and tough servos!