If you included a voucher, the voucher would have your social security number on it and the IRS would match your check to it.
The IRS will make every effort to match your payment to your tax file so that they can cash it. Watch your bank statement to see if it is cashed. If it gets close to tax day, (April 17th, 2018) and your check has not cleared, stop payment on it and mail a new check with your Social Security number on it, the tax year the payment should be applied to, and a letter of explanation.
