How do we find where service is covered??

There’s a difference between what is covered, and what is free.

What is free and included in your domestic plan is listed in your customer service summary in your myATT account.  
it may include North American roaming which means all of the United States, including Alaska and Hawaii, Canada and Mexico..  

* Currently Puerto Rico and the Virgin Islands are still roaming for free even though they have been sold to liberty mobile.  This may change.  
   Prepaid has much more limited roaming. Prepaid plans eight gigs and up roam free in all of North America.  There is paid Roaming and very select European countries.  No where else.

       This map shows where you would have coverage for free if you have ‘roam North America’.   

This map shows international coverage in orange, as long as you are using a compatible phone. Quality of coverage is not guaranteed as you are roaming on other carriers. In general you would expect to pay extra to roam in most of these countries.  

First of all, Guam is not the US, it’s a US possession. As for Canada or Mexico, they aren’t the US either. Whether or not usage in Canada or Mexico is covered depends upon on your plan. Most plans include Mexico. Most unlimited plans also include Canada. Bottom line, your plan dictates what is covered outside of the US but all plans include all 50 states. As for Puerto Rico and the Virgin Islands, ATT used to provide service there. They do not any longer. ATT sold their assets to Liberty. It is still unclear whether those locations are covered or not.

"What is free and included in your domestic plan is listed in your customer service summary in your myATT account.  "

The customer service summary in the myATT account states only that I have unlimited talk and text.  It does NOT specify where-- North America?  the US?  50 states plus DC?  nothing.

The map states that Puerto Rico and USVI are "off net" coverage, but they do NOT show up on the international map.

What is your plan name?  It's on the front page of your bill.   In addition to talk and text it should State how much data you have whether it's a certain amount of gigs or unlimited.

If you had roaming in Mexico or North American Roaming it would say so.  

It'll be there family share xxx something, Nationwide xxx something, Mobile Share value xxx something, etc etc.

It says Mobile Share Advantage

But the question still arises, how does one define "domestic"?

Okay...thanks to your info, I searched for Mobile Share Advantage and got to the FAQ that says "For use in the United States, Puerto Rico, and the U.S. Virgin Islands ("Domestic Coverage Area" or "DCA")." 

So, Domestic Coverage Area apparently includes the 50 states plus DC as well as Puerto Rico and USVI.    

thank you!

Puerto Rico and US virgin islands wireless services are no longer owned by AT&T.  While At&t customers are still roaming there for free, that may change without notice.  Do not travel there without checking with AT&T first, (and then expect them not to know)