I don't think there is a sure fire way to determine if they have been infected. You can examine the frames for signs of dead brood scale, etc.
Ethylene Oxide (ETO) fumigation is another option for treating equipment as well, if there is a facility near year. Keep in mind, that onces you have AFB, you can easily spread it with hive tools etc. so all your equipment will need to be sterilized.
If you are just starting out, I would not buy equipment with unkown history. If you are buying from someone you trust, and they know that the equipment is OK then fine. Otherwise stick with buying new.