Never rely on a computer code completely. Be sure to make hand calculations that confirm:

the code is correct (i.e. "bug" free), and that

you understand the equations and assumptions well enough to use the code wisely.

MODFLOW has been thoroughly tested by many people, so you need not test it. A code with less testing would require that you make hand calculations using analytical solutions and then set up the code input files to exactly simulate the situation represented by the analytical model.

A nearly exact match is required for validation. Finer grid cells/time steps, and occasionally a rethinking of the model construction to represent the proper conceptual model described by the analyatical solution, may be required to improve the match.