There is actually only one way: you need actual field measurements with some "proven" method (the most accurate is the good old leveling) that you ASSUME to be your ground truth. There is no way around this assumption. What you need to define are your accuracy metrics: absolute or relative, average deviations, maximal deviations to name a couple of them. In this case it is from great importance on how do you define the average elevation for a "pixel" based on your ground measurements.

You should doing a field measurements using Geodetic's GPS so that the results obtained more accurate