How can you tell if the vent tube on the differential is working and not plugged?

Usually if the cap on it wiggles around it should be good. If you are that worried about it, the whole vent just pulls out and you can replace it Here is one on ebay. http://www.ebay.com/itm/59-77-GM-Camaro-Chevelle-Nova-GTO-Firebird-Skylark-Rear-End-Vent-Tube-NEW-/231855646519?fits=Model%3AFirebird

Okay thanks. The cap does wiggle so I'll leave it alone. I've got enough other things to spend money on in this restro lol