The engine will visually shake and move when you rev the engine. You should be able to see the engine mount on the passengers side. It's located right under the oil filter. Run your finger under the oil filter and see how much space there is between the filter and the mount. If you can't easily get your finger between the filter and the rubber mount, it's probably worn to some degree. If the oil filter is touching or pressed against the mount, then you'll know it's sagging too much.

With broken or sagging motor mounts, you'll also get some vibration through the steering wheel. I noticed it more while turning than when going straight.