on the axle tube itself, left side between that u-bolt thats for the anti-sway bar and the coil helper is the vent line, coming straight off of the axle. the only way i know to test it would be to remove the fill plug, wrap a rag around a air-blow nozzle and lightly/gently put pressure into the diff, air should come out of the vent line. the top end of the vent line is clipped to the frame rail and can be easily removed/reinstalled.

__________________
blue streak ( R-140) build-->https://flic.kr/s/aHsmBWXRwL
'93 s/c 4.0 4x4,"the green machine" (retired-r.i.p.)
'92 reg cab longbed 2.3 "polar express" (stolen)
' 97 reg cab 3.0 auto ($500 project truck)-rolled
'94 2wd longbed 2.3 manual trans-"R-140"
pictures--> https://www.flickr.com/photos/154582...h/37156884305/