If you deduct your property taxes as an itemized deduction on schedule A, you must reduce the amount of your property tax deduction by the amount of the STAR rebate.

If you don't know the amount, check your bank records (if you deposited the check).  You may be able to call the state tax department and get them to tell you the amount of the check.

If you don't reduce your deduction, and are later audited, you will owe back taxes, interest and penalties.

It is your responsibility to keep track of all your important tax records.

If you don't itemize deductions and don't deduct your property tax, then the STAR rebate is not taxable income and does not require any adjustments.