In the xtivreg world, for example, while Sargan-Hansen testing helps assess instrumental variable efficacy, researchers also typically confront the question about whether a FE or RE specification is appropriate. Such a question was posed recently on the StataList (here) and an initial response echoes prior suggestions that focus on the Mundlak approach. Mundlak advances an alternative approach to the Hausman test insofar as, unlike the latter, the Mundlak approach may be used when the errors are heteroskedastic or have intragroup correlation. (An earlier discussion goes into greater detail.)