The IRS will look to match the income you listed on the self-employed/business section to the income reported to them by the individuals and/or companies that paid you. Only when the income you report to the IRS is less than the income reported to them by the individuals and/or companies will the IRS contact you for an explanation.
If any one company or individual paid you $600 or more than they have to issue you a 1099-MISC form, per IRS regulations. If the company or individual paid you less than $600 then you will not likely receive a 1099-MISC from them because the IRS does not require them to because it is under the $600 threshold.
If you are saying that all your income is undocumented, i.e. no 1099-MISC forms, then the proof of your income becomes the tax return itself. In this case, keep good records that show how much money you made and how much money you've spent. Good records is a relative term, but usually would include receipts, sales slips, mileage logs, bank statements, etc...(this is not an exhaustive, just an example to get you started).
