does the engine actually sag?

i used to have a rough idle issue where the rpm would be bouncing around in D and sitting at traffic lights. but more recently, i noticed that when i put the car in P and let the car idle, i can feel the car jerking around which makes me suspect the motor mounts are shot. it's a 97 E420 with 109k miles.

is there a way to test the mounts before replacing them? btw, the motor mounts seem like something i gotta pay a shop to replace..

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Current cars:
2001 MB E55 AMG
2003 Honda S2000
2000 Honda CRV

Gone but not forgotten:
2004 BMW 325i 5sp sport
1999 BMW 540i sport
1997 MB E420
1995 BMW M3 coupe