Generally it's a specific guage that mounts to the head, which you zero on the round part of the cam, then you rotate the cam and see what it's maximum lift is... (the lobe is now pointing up towards you). The duration is just how many degrees the lift even lasts... generally I think it's measured from the first even of lift to the last. But I seem to remember something about it being measured only after a certain amount of lift has occured... like .05 or something... but I could be totally wrong on that...
Search google for "measuring lift/duration" or something along those lines...
Never mind...
Here's a link that has the tool (dial indicator)...
http://www.gearheadtools.com/16376.htm