You seem concerned about a vibration -- motor mounts are not really designed to hold the engine so firmly as to dampen vibration; instead, they're designed primarily to support the weight of the vehicle, and hold it in place during accelerations, etc. Ironically, absolutely rigid mounts would actually transmit MORE vibration to the body that you would feel -- mounts have some flexibility to mask such vibrations.

Signs of a bad motor mount (or mounts) include the engine sagging, so that you can't slip your finger under the oil filter against the "frame rail" (not really a frame, but the concept fits). Or the engine shifting so much that the fan hits the shroud just in back of the radiator. Or, just gently rev the engine, in gear with your wife's feet firmly on the brake pedal, and while you look at the engine rock side to side. If it's excessive, that's bad.

But for vibration, I'd look elsewhere for a cause.