Bad valve stem seal -- how to tell

http://www.fordforums.com/f655/5-0-uses-oil-rings-valve-stem-seals-how-tell-103279/#post1080284

Well, there are a couple of ways to tell. If you start the engine up
and it puffs out smoke, it's the valve seals because you are burning
the oil that has leaked past the seals when the engine was off. If you
slow down by downshifting and when you accelerate again, and it smokes,
it's valve seals. Why? Because when you deaccelerate by downshifting,
your throttle plates are closed and you have a lot of manifold vacuum.
That vaccum sucks in the oil past the valve seals. Then when you
accelerate, the increased air/fuel charge sucks it into the combustion
chamber and it is burned.
Unless the maintenance was completely ignored, I'd guess that it is
valve seals. They will deteriorate with age while a piston ring will
not.