Triggers and Function.

How can I identify if a column is actually part of the UPDATE that fired
the trigger.

For example

If I have a table with three columns col1, col2 and col3. The types
don't matter to the question but may to the answer.

I run "UPDATE example set col1 = 'NewVal', col2 = 2 where col1 =
'OldVal';"

A BEFORE TRIGGER can test OLD.* against NEW.* and see col1 has changed
col2 may or may not, OK to track what has actually changed, but how can
I find out that col3 was not part of the update?

I need to know if the client has updated a column, even if it still has
the same value.

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