How to manually validate a gpart layout?

[Michael Dexter]

Hello all,

The kernel will report this at boot if a primary GPT is invalid:

GEOM: da1: the primary GPT table is corrupt or invalid

How does one manually validate the table with gpart? I sense that 'gpart 
recover' will perform enough of a test to say that recovery is not 
needed but it is not clear if this is the same test that the kernel is 
performing.

Thank you,

Michael

(And yes, I asked Michael W. Lucas if it is covered in his new book (GO 
BUY IT!) and alas, no, it is not.)