Best method for checking a warped intake manifold?
 

I am going to the junkyard to look for a GMC, 1990, c2500 5.7 liter, t.bi. aluminum intake manifold and was curious to know as much as possible when it comes to finding one which is not warped.

I have read quite a bit on here and other forums, with differing points of view on how to check these intakes for signs of them being warped or twisted.

Some say use a straight edge along all the intake ports with a feeler gauge, other say just place a straight edge across adjoining intake ports and check them that way with a feeler gauge. Some even suggest placing a bright flashlight inside the lifter valley of the engine to check for any light which you might be able to see once you place the cleaned intake manifold back on the cleaned intake ports of the heads.

In checking for an intake which might be twisted, I've read about taking the cleaned intake manifold and placing it on cleaned intake ports of the heads and moving the intake side to side at opposing corner to see if it rocks back and forth.

One more question, are all these t.bi. intake manifold identical from 1987 to 1995?

Thanks ahead of time for any help or suggestions.