The IRS runs a comparison of income between what you file and what was reported. If the income you report does not match the IRS records, they will send you a letter with the amount owed and any interest and penalties that have accrued during that time (the time period starts 4/15 of the year the return was supposed to filed). The IRS can take a year or two to send you that letter. You will continue to accrue interest and penalties until you pay your balance in full.
