How can one to determine what official clearance they have been granted?

[nukbee]

I have read the "Information Security Information Defined" http://www.nukeworker.com/forum/index.php/topic,2546.msg21051.html#msg21051; however, I still don't quite understand a few things.

I would like to know how to "prove"/verify that one has a valid clearance and more specifically which clearance(s).

I also would like to have/see a definitive table to correlate the Nuclear Clearance equivalents to non-nuclear Government Clearances. 

I am specifically interested in knowing about the process to request this type of clearance documentation or proof from an employer or through the Government via the Freedom of Information Act.

Any pertinent information or resource site is appreciated.

Thanks in advance!