If you want to use a vacuum gauge, you tee into the line, run the gauge into the cab and drive the thing. Since smogs are done on a dyno, you need to see what vacuum (or if you get vacuum) under realtime load.
If you just want to see if the egr valve is working, start the truck, apply manifold vacuum to the valve(not the transducer, the valve) and the engine should start missing horribly or die. You can suck on the vacuum line if you want, it works the same.
Usually, I do the apply vacuum test first, to see if the valve diaphragm is good, and if the tube is plugged. Once you have determined that, move on to testing the vacuum under load(the first one i mentioned). Lastly, if your truck has it, test the transducer valve. it is the flying saucer shaped valve above the EGR valve with vacuum in, vac out and pressure in from the exhaust on the bottom.
Hope that helps.
