How can you tell if egr is bad?

If you are talking about the EGR valve, then take a piece of rubber tubing and plug it onto the EGR valve vacuum port and using your mouth pull a suction on the tube. The valve should actuate (pull in) and if you hold it with your tongue by stopping up the end of the tubing, it should also hold it's vacuum. If it loses it's vacuum or doesn't actuate it is bad.