The IRS does not "track" contributions and earnings, the IRS can only determine the taxable amount by manually reviewing all of an individual's Forms 1099-R and 5498 over the years, the same information that is provided to the individual. Assuming that there have been no other contributions to Roth IRAs, the information on the Forms 1099-R and knowing whether or not the individual was under or over age 59½ at the time of the distribution from the Roth 401(k) should be all that is necessary to determine the taxable amount of the distribution, as long as the Roth 401(k) administrator has provided all of the information on the Form 1099-R that they are responsible to provide.
