You are responsible to report all taxable income regardless of whether you receive a form, but it is the forms the IRS checks using their computers.
Restitution payment can mean different things, but if you are being reimbursed for something you lost and you never reported the loss as a tax deduction, then the restitution may not be income but rather reimbursement (not taxable.)
This might be something worth consulting a professional about, especially if the amount is large.
