I've seen a POA a few times. In every case, the PA got its own confirmation/legal opinion that a POA was valid or not. (This may have included other practicalities, such as other known evidence about a participant's disability, etc.) In all cases, the PA was clear that lack of authenticity of the POA would mean "don't do anything", except that the plan's appeal process may be triggered by the submission of a (proposed) POA. If the POA was determined to be valid, the same legal advisor would describe whether the POA has limitation(s) within the plan context. (IMHO, this last part is essential, and is evidence that the legal advisor reviewing the POA should be an ERISA attorney.)
As the actuary, I give my opinion only when requested by the PA, but always defer to opinion of the legal advisor.