When you changed the sparkplugs, did you notice one that looked quite different from the others ( carbon build-up )?
One way of checking for a dripping injector would be to observe a pressure gauge reading ( tapped into the fuel rail ).
The best method would probably be on a testbench.
Depending on how much $$$$ you want to spend, you could even have the injectors cleaned, flow-tested and ultimately "blueprinted".
P.S. Maybe Gilly or somebody will chime in here.
2007 C 230 Sport.