There is no single person at the IRS that “knows” what you’re doing when you cheat on your taxes.
You should know, however, that the figures in your return will be checked by a computer against:
A) your previous returns
B) figures reported to the IRS by your employer, bank, broker, etc.
C) millions of other returns of people who share your profile (age, marital status, income bracket, occupation, etc.) and if your figures are inconsistent (enough) with the range the computer expects for someone of your profile, you may be flagged for an audit and that’s when you can get screwed, because it is then you will have to produce documents (receipts) and justify your deductions and other transactions that must include a paper trail.
If you are unable to explain or document any and everything the auditor asks for, you can get hit with payments and penalties. The payments (back taxes) are not usually the problem, but the penalties are usually painful because they can run high very quickly if you are not able to make full payment in the limited time that they give you.
A simple math error on your return is usually identified (and you’re notified) within a year of your filing. Most audits, if they are going to occur at all happen within 3-5 years, but that is not an absolute cut off date.
If you’re going to fudge some numbers, be reasonable--don’t get greedy.