Here's a decent website regarding the pre-employment background check.

http://www.privacyrights.org/fs/fs16-bck.htm

Here's the official NCIC website...

http://www.fas.org/irp/agency/doj/fbi/is/ncic.htm

When I was assigned to my agency's Investigations Unit, I used to conduct background investigations (BI) for peace officer applicants. The pre-employment BI process is required by the State of California and has very strict guidelines.

http://www.post.ca.gov/forms/2-251-phsPeaceOfficers.doc

I always gave the same piece of advice to would-be cop applicants:
Be honest!

I would also give the same piece of advice to any person seeking employment where the public's trust and/or safety are at stake.