You would be removing the plug wires and measuring them end-to-end.

Your car is at just the right age & miles for the plug wires to be getting a touch flaky. About 7 years is how long the M112/M113 wires seem to last. Idle quality degrades first, though it takes a while to throw a code.

I had the same symptoms on my 2001 E320 - an occasional, single stumble at idle. The problem was somewhat correlated with wet/humid conditions.

Replacing the plug wires eliminated the occasional stumble.

- JimY