how do you know it is bad if you dont know how to test it or the EVP? im not trying to be a jerk, but rather to perhaps get more info about your problems and codes you might have.
the egr inputs spent (inert) gasses at cruise. this cools the chambers and improves fuel economy. while the egr is active, the puter dials in a lot of timing, as the egr gasses dilute the mixture and make it less volatile.
the puter sends a signal to the EVR to activate the EGR valve. at idle pull the vac hose off the egr - note if it has vac - it should not.
there is more with the EVR, but since you mentioned EGR specifically, ill stop (besides Michael, JR, et al explain this stuff much better than i).
as an aside, sometimes simply cleaning the pintle in the EGR helps - it can get coked up and keep the egr slightly open when it should not be. conversely, it might not open as far as it should when it should be open all the way, which could lead to detonation.
good luck.