actually,
you wont see a message (like when u use a dbms_output).
you will see your text in the error stack along with the number you have entered in raise_app_err
process STOPS if it encounters a raise_app_err.
even if u use dbms_output u will not get a clear message as
shown...

----------------------------------------------------------------------

mag@itloaner1_local > ed
Wrote file afiedt.buf

  1  declare
  2  a number:=í
  3  begin
  4      if a<>1 then
  5     DBMS_OUTPUT.PUT_LINE('before raise application --- U HAVE NOOOT ENTERED 1');
  6             raise_application_error(-20000,'should be 1');
  7     DBMS_OUTPUT.PUT_LINE('after raise application----- U HAVE NOOOT ENTERED 1');
  8      ELSE
  9     DBMS_OUTPUT.PUT_LINE('U HAVE ENTERED 1');
 10      end if;
 11* end;
mag@itloaner1_local > /
Enter value for nu: 1
old   2: a number:=í
new   2: a number:=1;
U HAVE ENTERED 1

PL/SQL procedure successfully completed.

mag@itloaner1_local > /
Enter value for nu: 2
old   2: a number:=í
new   2: a number:=2;
before raise application --- U HAVE NOOOT ENTERED 1
declare
*
ERROR at line 1:
ORA-20000: should be 1