How can you tell a Cam is going bad on a 1992-S-10 4.3 v-6 Vortec? And also, what all causes low vaccum besides leaks?

low vacuum can also caused by incorrect timing worn rings, lean or rich fuel mixture, plugged catalytic convertor, check out this link for the full vacuum story.
http://users.bigpond.net.au/ergoff/vac1.htm
to check for a worn cam remove the valve covers and measure the height of each valve fully closes and fully open. i can't remember what the lift is supposed to be but generally it's camlift x rockerarm ratio.
good luck