I've ever tested a rad cap, but I read where you put it in some water heating it up. Drop in the cap and watch to see it operate when hot enough..

Remove the radiator, install the cap, and apply measured air pressure to the radiator hose bibs.  Submerge the cap area of the radiator in a tank or bucket of water.  Increase the pressure gradually and note when air starts coming out of the overflow line.  There should not be ANY bubbles coming from anywhere at lower pressure.  If you have a 15 lb cap, bubbles should start coming out of the overflow line around 15 psi.  If you have bubbles coming out in other places, find the reason and correct it.