If your check is signed by Uncle Sam, then your salary is likely public information, as is everyone else's. It may take getting through some red tape, but the information is available to you. Check out your state’s website. (State comptrollers often manage the records for such matters.) Or if you work for the Federal government, visit the website for the US Office of Personnel Management. If you work for a company, public or private, research salary ranges at career research sites like payscale.com or salary.com. Also, get familiar with trade-specific websites and trawl message boards for more candid chatter about industry salary standards.
Chat up your mentor or a trusted officemate with a higher title. Pick her brain on how to approach your boss for a raise, then quiz her on the appropriate salary for your level in the company. Don't try pinning her down to one hard number: ask for a range. Figure out where you fall in the spectrum.