I can't remember where I bought it, but I have a stethoscope looking thing with a long piece of hose about 1/8" ID. I simply put on the stethoscope and probe around connections and places where there might be a vacuum leak and listen for the leak.

This is a fancy and more effective version of the piece of hose that I watched my Dad hold to his ear and probe around with looking for vacuum leaks.

Good luck,