How to determine if idling vibration is caused by spark plugs or engine mounts?

Don't just replace spark plugs because a customer says the 'car is shaking' at idle. I usually first check if the vibration is more noticeable when the engine is cold, hot, in Drive with the brake pressed, or with the A/C on. I scan for misfires, then check if the engine mounts have collapsed, if there are vacuum leaks, and if the throttle body data is normal. Once, a customer replaced two sets…