Can I trigger an action from a coalesce ?

I have a case where if a value does not exist, I am going to use a default,
which is easy with coalesce. But I would like to warn the user that a
default has been supplied. The default value is reasonable, and could
actually come from the source table, so I can't just check the value.
I'd like to do a raise NOTICE, if the default portion of the coalesce fires.

Anyone have a good way to accomplish this?


-- 
"They that would give up essential liberty for temporary safety deserve
neither liberty nor safety."
                        -- Benjamin Franklin