How to find out if row was modified by EXECUTE UPDATE ...

Hello.
Doesn't this work in plpgsql functions?

EXECUTE 'UPDATE mytable set ... ...'
IF FOUND THEN
       do stuff....
END IF;

It seems it always evaluate to false in the if statement, Isn't  found
used to see if an update has modified rows?

Are there any alternatives to selecting the row and see if the values
are set to see if the update worked?