If you don't remember what the check was for you will have to call the state tax office. They should be able to tell you if they sent a check.
The freeze credit and relief credits are only taxable income if you itemize your deductions and deduct your property taxes.
If part of the check that is a credit for 2016 taxes, you will reduce your current property tax deduction by the amount of the credit. It's not income, and if you don't deduct property taxes then you don't have to do anything.
If part of the check was a credit for 2015 taxes, then it is "other income" at the bottom of the income page, look for "reimbursed deductions" -- but only if you deducted property taxes on your 2015 return. If you didn't deduct the taxes in 2015, then the credit is not reportable income.