If that, let keep reading :D Officialiphoneunlock.us provide INSTANT checking service. You can check it out at: http://officialiphoneunlock.us/check-iphone-imei
For AT&T blacklist check. You just need to following these simple steps:
1) Click http://officialiphoneunlock.us/check-iphone-imei, type your IMEI and choose service: "AT&T Blacklist check"
2) If your IMEI is valid, the button "Check IMEI" will appears, click it and Login or Register.
3) After logged in, you will be redirected to payment page, here we supported PayPal. You have to complete your payment to continue (Just $1)
4) After payment already successful. Access to your Client area -> Order process tracking -> Click "Send check request" and you will receive information like the image below