Timing question

[BamaDave]

I bought 350 a long time ago when I was a kid for my truck. I was a mild build with a more than stock cam. I don't know the time for this cam. How would I make sure timing is right. I am rebuilding my Holley and trying to fix any vaccum leaks I can find.
I bought a compression testor but really don't know how to go about checking the comp. Just pull plug boots, put testor in each cylinder, turn it over?
Doing all of this because this thing is running REALLY rich and I can't lean it out without the truck dying.