testing for DEFAULT insert value in ON CONFLICT DO UPDATE query

Hi

Is there any way to tell if a conflicting row in an multi-line INSERT
used the DEFAULT directive?

I would like to be able to upsert a bunch of rows and only UPDATE the
conflicting rows where the value set was not new - the way I do this
for NULLable columns is to just write NULL in the INSERT subclause for
the columns that I don't want to set and use

SET column=CASE WHEN EXCLUDED.column IS NULL THEN tablename.column
ELSE EXCLUDED.column END

(or COALESCE(), if you prefer);

however for NOT NULL columns with a default, I don't know how I can do
this. I was hoping for something like an "IS DEFAULT" test but that
didn't work. I can't just test for the default value itself because
there might be times when I want to update the value to the default,
overriding an existing value.

I also can't simply exclude the column from the insert because for
some rows I _will_ be setting the value.

Am I missing something obvious, or am I going to have to change the
column to accept NULLs in order to make this work?

Thanks

Geoff