imei checker?

Need to know if there is a way to check imei on at&t iphones and smartphones other than calling the carrier directly to see if they are on someones account or blacklisted. I buy iphones and smartphones from time to time but i have gotten burned lately and so if anyone knows of a reputable service free or paid, i would appreciate it. Tmobile has an imei checker which will tell u if a device will work on there network but sometimes it can be blocked for at&t and not tmobile so its no entirely accurate for my purposes. Any help is appreciated and sorry if this question has been asked. i used the search option but couldnt find any relatable posts. thanks in advance.