If an extension library is loaded during pg_upgrade, can it tell?

Hi,

Is there any distinctive state that could be checked by extension code
to determine that it has been loaded incidentally during the operation
of pg_upgrade rather than under normal conditions?

PL/Java ordinarily checks what version of its schema is around, but
that may be premature while pg_upgrade is doing its thing and the schema
might not be all there yet.

-Chap