Unfortunately PL/SQL doesn't have IF EXISTS operator like SQL Server. But you can do something like this:
begin
for x in ( select count(*) cnt
from dual
where exists (
select 1 from courseoffering co
join co_enrolment ce on ce.co_id = co.co_id
where ce.s_regno = 403
and ce.coe_completionstatus = 'C'
and co.c_id = 803 ) )
loop
if ( x.cnt = 1 )
then
dbms_output.put_line('exists');
else
dbms_output.put_line('does not exist');
end if;
end loop;
end;
/