select statement details

I'm trying to determine if a select statement:
1 - causes execution of a Volatile function
- or -
2 - causes execution of a nextval function (same/similar as #1 above?)
from within tcop / postgres.c ??

Things like QueryIsReadOnly imply that select nextval('some_sequence') are
read-only ... which is not read-only in the sense that the database is in a
different state after the nextval runs.

I've been looking through PortalDesc & NodeTag, but haven't been able to find
anything...

suggestions?

If nothing exists, I'd need to create something.
Any suggestions where to start?