When the engine is keyed on, the ECU goes through a POST. That test will check for a problem with the switch. If the SES fires with a EGR code, then you know WHY the code has fired. If it happens at start up, it's the switch, if it happens after a drive, then it's the valve.

I can only speak from my expereince, so here's what's up with the above.
The SES light would come on after 4-5 minutes of driving at somewhat consistent speed (freeway driving). After clearing the code, driving back and forth the 7 minutes to the office, no SES. Back on the freeway, 4-5 minutes, SES. Having gone through the FSM tests and then some, I found the EGR valve was good (held vacuum for longer than required), the vacuum lines held required vaccum, checked the 2 electrical wires to the EGR vacuum solenoid all the way back to the ECM, finally re-read all the instructions again and found the solenoid did not preform as it was supposed to.
Now the down side is - - find a EGR vacuum solenoid - I have spent hours with the local a/p stores and their warehouses - there is another post regarding this - Standard is still up in the air if they are going to manufacture again, B/W is out of stock on the part number, as is Blue Streak and AC - J&D, if they have a used one, is "somewhere around $125.00".
All that said, re-read the FSM and all the test proceedures before throwing $ and time only to find the valve was ok and it's the ECM or the solenoid or a broken wire that's the culprit.
Good luck
Subman