One of the easiest ways to check for a leaking EGR tube is to try to wiggle it after starting the car (engine cold -- you don't want to touch a hot one). If it has a leak, you will hear a changing hissing sound as you wiggle the tube. Another check is to spray starting fluid on the EGR tube. If that causes an increase in RPM, you have a leak. Both of these checks are easier than installing a blocking plate, which must be on the manifold end of the tube to do any good.