How do I find out what Hidden Oracle Parameters there are?

Well, it's simply really...you run the query below:

SELECT
a.ksppinm "Parameter",
a.ksppdesc "Description",
b.ksppstvl "Session Value",
c.ksppstvl "Instance Value"
FROM
x$ksppi a,
x$ksppcv b,
x$ksppsv c
WHERE
a.indx = b.indx
AND
a.indx = c.indx
AND
a.ksppinm LIKE '/_%' escape '/';

The query above will display all of the hidden parameters within your Oracle database. Please take care when setting any hidden parameter. The only reasons to set them is at the advice of Oracle support. However, I would recommend having a play around with them on a DEV environment which is OK to be trashed