
Quoting
Bubba Jimmy
1. You are required to report your income whether you receive a W-2 or not. The IRS provides form 4852 "Substitute W-2" for this purpose.
2. Employers are required to withhold social security and medicare taxes, and depending on your W-4 specifications, perhaps income tax as well. The amount you received from the employer should be net of these amounts. I have taken the position with the IRS that the amount paid to the employee was net of taxes and that the tax money was still in the employer's bank account. The IRS will, under the right circumstances, accept this argument and go after the employer to collect the taxes.
3. To take such a position you would need to use bank deposit records, a log book of actual receipts from your employer, or other documentation that would support how much they actually paid you.