How can you tell they are bad? I feel slight vibration at idle, but goes away as soon as it's in gear revved.

 

when you accelerate you'll hear a strong "thud"!!! thats the engine jumping higher than it should!!! but the total mount might not be broken just the bolt(insert) that connects the base of the mount and the top of the mount. heres a screwy diagram:
<b>}</b>=(top of mount); <b>{</b>=(base of mount); 0=(connecting "bolt")--------->

[Engine]<b>}0{</b>[chasis]

the (0) part might have snapped and is easy and inexpensive to replace!!!