With your SIM card in a dummy phone, call AT&T and ask them to block data from being received on your phone, because you don't want to be charged extra for something you're not using, or may accidentally activate. They will block data for you for whatever reason as long as your SIM is not in a smartphone/internet phone because they can tract and know exactly what type of phone you have your SIM in.
Now you can switch your SIM into your smartphone, and only use wifi. On your next billing cycle, you can check online at the AT&T website for your account info. Here you will be able to see the data plan option, there shouldn't be any.
But if you want to double check, click [Add/change features].
Scroll down to [Web, Text, & More]
Under [Mobile web packages]
You should see the [Data opt-out] option selected
Enjoy