you can check by blowing into the vac line going to it... if it lets air out its bad. Thats how i diagnosed my stocker going bad. But, as i more recently discovered, even if it holds air in that situation, if it doesn't hold vacuum it also causes problems. I figured the diaphragm tore and i'm not sure what would make it not hold vacuum but be fine to hold boost... but regardless, you need to check for both.
