How do employers typically verify a person's backround to ensure the person has:

The experience they claim they have?
A degree from the school they claim to have attended?
A previous salary consistent with what they actually made?

Companies like Kroll do background checks.

http://www.krollworldwide.com/services/screening/background_investigation/

Often the salary data can be validated by calls (and an exchange of
private information) to the HR group of your previous employers. 
Salary information is released, but bonuses might not be included.