A'B'C' + A'B'C + A'BC' + A'BC + AB'C' + AB'C

Now using a K-map and using just the boolean postulates, I simplify the expression to just:

A' + B'

But my question is, how do you compare the truth tables of these 2 functions to see if they have the same output? I don't see how just because the literal C nulls out after all the simplification.