have a copy of the check front and back available. call the number on the notice. the IRS should be able to track the payment based on the info it stamps on the check. They may request you send in a copy - send front and back. if so, get an address - it may be different than the address on the notice. use a method where you get proof of delivery. do ASAP since the IRS generally only gives 60 days to respond.
