Engine Mounts

[99SL5]

I took my car in a couple of days ago for a clunking noise when accelerating in first gear. It was coming from the center of the car so I thought it was either the transmission or play in the drive shaft. The dealer thought it was engine mounts. The repair bill was $750 and it I haven't noticed any difference. I have a 99 SL500 with 47k miles on it. Is my car at the age or mileage when these need to be replaced? I have read other people replacing them on 20 year old cars with over 100k. The service advisor said my engine was sagging. I am wondering if he was just saying that. Can you determine that the mounts need replacing by just looking at them (they seem pretty much covered).

I am going back Tuesday to see if they can determine the cause of the clunk and I have requested my old mounts. What should I look for in faulty mounts? Obviously the rubber has to be in good condition (no cracks), but is there a certain thickness or anything else I should be looking for?

Thanks for any input!