It should say on your AT&T account online.
If you log in to your profile, under the top heading "myAT&T" go to "Profile" and then "Check upgrade eligibility."
If your eligible it should read:
"Congratulations! You are eligible for an AT&T NEXT installment plan upgrade. Or, get a standard upgrade today with a 2-year service agreement and a $36 upgrade fee."
If not it should say a date that you're eligible. Sorry I can't give your the latter format since all my lines are off-contract.