The following is an actual user-submitted question:
Q: I just wanted to know if anyone can find out about your background without your knowledge. Do they need your Social Security number for a background check? If so, can this be used against you when applying for a job?
A: These are very good questions. We will answer them in order. First, anyone can conduct a background check without the subject's knowledge. There's no way to know if someone has performed a background check on you. Secondly, a Social Security number is not required for a basic background check that includes criminal records. Only a first and last name are required, and sometimes a city or state. Background check services use only publicly available records, including marriage licenses, deeds, bankruptcy files, and arrest records. It's important to note that the searches done using a background check service are different from official requests for records from the state. State requests may require the applicant's permission (and possibly an SSN), depending on state law.
The answer to your last question also depends a lot on which state you live in. Some states do not allow individuals with criminal convictions to be employed anywhere, whereas others may have protections against employment discrimination. California, for instance, bars employers from requesting the information in most cases, and prohibits discrimination based on prior arrests. Unfortunately, such discrimination is hard to prove.
One thing you can do is to sign up for a well-rated background check service such as Intelius, Exploreos, or Net Detective and look yourself up. That way you'll know what records may be out there, and you can be prepared to answer any questions. After all, not all records turn up in those searches, and some records are incomplete. You can also conduct an Internet search to see what the laws are governing job discrimination against individuals with criminal records in your state.