The original question was "how can I tell if my crank was turned?" He didn't ask how to build a precision racing motor.

Within normal temperature ranges, plastigage will not vary enough to make a difference. It is more than accurate enough to make that judgement. If the crank is ground, it will be .010, .020, or 0.30. You don't need an inside mic to figure that out.
It is for people who don't know how to read an inside bore gauge or mic.
Eric,

If you read that again, you might see why someone would think you were putting them down with that statement. Just a thought.