A guidance document for establishing the validity of specific models has been published in "Recommended Practice 6: The Use of Modelling in Inspection Qualification" by the European Network for Inspection and Qualification:
European Network for Inspection Qualification (ENIQ) - Recommended Practice #6
This document notes the importance of establishing the "physical basis" and "regime of validity" of each model. Approximations used in the models and their limitations should be identified.
Justification of the validity of model results can be established by comparing the model results with reliable experimental measurements, by comparing the results of the model with those from another independent model (for example compare an Analytic Solution with a Finite Element Numerical Solution), or by setting up a complex model to solve simplified situations with known answers. An example of the latter might be to set the model up to approximate a uniform field source and verifying that the electromagnetic penetration into a thick conductor is equivalent to that governed by the simple skin effect equation.