Here is way to list all hidden parameters and their current session/system values. You should be careful when changing these parameters and is highly recommended to make change only after consultation with Oracle Support.

set lines 150
set pages 200
col parameter_name for a30
col parameter_desc for a80
col param_sess_value for a10
col param_system_value for a10

select t.indx,
       t.ksppinm parameter_name,
       t.ksppdesc parameter_desc,
       t1.ksppstvl param_sess_value,
       t2.ksppstvl param_system_value
from   x$ksppi t,
       x$ksppcv t1,
       x$ksppsv t2
where  t.indx = t1.indx
       and t.indx = t2.indx
       and t.ksppinm like '\_%' escape '\'
order by t.indx