If the distribution took place in 2018, you will receive a Form 1099-R for 2018 that you will need to include on your 2018 Tax Return. 

It is possible that the distribution took place by 12/31/18 even though didn't receive the check until 2019.  If you haven't received a Form 1099-R and are unsure when the distribution took place, you may want to contact the Financial Institution where your account is located to verify.