Determining caller of a function (due to a cascaded FK constraint?)

Greetings,

For reasons a little too complicated to get into off the bat, I am
wondering what the most effective way is to determine by whom or how a
particular action or function call was initiated. To shed more light, I
want to have a trigger that will copy some data from a table in one
schema to an analogous table in another schema every time a record is
modified UNLESS the modification is the result of a cascaded foreign key
constraint. My hunch is that the answer somehow includes using data in
pg_class and/or pg_proc, but I haven't quite pieced it all together.
Does anyone have any recommendations on how to go about this?

Thanks,
JL