You can tell if the motor mounts are bad, by looking at the engine and having someone step on the gas and release the pedal very quickly. If the engine is rocking side to side you have bad mounts, if the engine stays center in the engine compartment, the mounts are still good.
Before having them replace the mounts ask them if they think the mount is the problem. Worse thing is if you spend all that money and the mount wasn't the problem, but they will tell you that you told them to replace the mount, and they did what you told them to do. Have them drive the car and tell you what they think is the problem and get a cost for the repair. Never give them an open check to fix the car.