How do I tell if my AT&T iPhone 5 is unlocked?
I believe you are unlocked.

I just conducted an experiment for you.  I took my T-Mobile SIM card out of my iPad and put it in my locked AT&amp;T iPhone 6.  It immediately told me "SIM Not Valid. The SIM card that you currently have installed in this iPhone is from a carrier that is not supported under the activation policy that is currently assigned by the activation server."

That is what you'd get if you were locked. Congrats: It seems they did indeed unlock your iPhone.