You can easily watch it leak on a dyno. The curve typically dips down then goes back up. To demonstrate, this is a stage2 car look what I'm talking about is shown around 4000-4400rpm. With a properly set up BPV that dip would not be there.
http://i529.photobucket.com/albums/dd340/Jay04WRX/4427A362-44A9-41D1-8350-54077341C237-1961-00000124F93E6F81.jpg
Before and after is just tune only, so you can ignore that if you'd like. He added a Forge BPV down the road, good thing is a re-tune is normally not necessary.