A friend loaned me his .308 and a Lee collett neck sizing die but no instructions. How do you know if your efforts have been successful. How do you know if you have properly neck sized your brass? I've looked everywhere online and found little to no info.! Suggestions?

The collet die squeezes the neck onto a mandrel. set the die so the up stroke of the ram bottoms out before the press cams over.
lee states that the die can be damaged if you cam over and the also state to use 25 pounds of pressure?
the mandrel size will determine how much you can resize the neck. if you watch the die the sleeve will slide up into the die that travel is about as much sizing as your going to get.
Measure the inside diameter of a fired case. run it through the die and measure it again. that will tell you how much you have sized the case including spring back of the brass. they usually will measure some where between .304" and .307" for a 308 case.
Dave

You can find Lee's FAQ here http://www.leeprecision.com/cgi/faq/index.cgi Click on "dies and die questions" There are 3 clickable "collet" topics.

You can tell if you've sized the necks "enough" by trying to seat a bullet by hand - if it goes in you didn't do enough, if it won't go in you're in good shape.