Bent stems are easy to identify- - - -during the regrinding process they will wobble against the grindstone and make a distinctive sound. I could tell when a student had a bent-stem valve in the grinder from halfway across the school shop just by listening to the sound the grinding machine was making. The margin will also be an uneven width if somebody insists on continuing to grind a valve with an obviously bent stem. After grinding, there should be a minimum of 1/16" margin on an intake valve or 3/32" on an exhaust. (.060" intake, .090" exhaust) . Measure the stem diameter where it enters the valve guide with a micrometer. As long as it's within .001" of original diameter, it's probably OK, but be aware that any wear at all from top to bottom will cause a slight side to side wobble in the guide, because guides must be reamed to accommodate the top of the stem for the valve to be able to be inserted into the guide. All these things were common knowledge back when real mechanics did their own machine work. Now we're dealing with the lack of knowledge created by around three generations of parts swappers who have no clue about how engine machining is done.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!