They mostly rely on computerized matching of W2 and 1099 forms. Just as long as what you put down as money received has the same numbers as the forms they received, you can then use the most favorable interpretation of the laws in your deductions and calculations, provided you tell the truth and act in good faith. If your numbers add up, they are very unlikely to be challenged.
and you really don’t need to file as long as you don’t owe money