I want to check that the EGR Valve is operating within specification. How can I tell if it's good or bad?

There are three failures:

1. The diaphram
2. The vacuum control signal
3. Gunk

1. Easy. Apply vacuum to the diaphram with a vacuum pump with a gage attached. Engine should try to die. If it doesn't then suspect 1 or 3.

2. At operating temp, check for moduation in the vacuum control signal.

It is possible for the control vacuum to stick. Removing the hose and plugging should make the engine idle smooth.