Note that Atmos is only available from Amazon Prime if you pay for the no-ads premium subscription.

https://www.theverge.com/24071417/amazon-prime-video-no-dolby-vision-atmos

Thanks.

What is the best way to test Atmos then?

Apple TV has everything in Dolby Atmos. If you don't subscribe to that then Netflix has a lot in Dolby Atmos. If you don't subscribe to either then you'd have to find a test file to download somewhere. 

Disney+, Apple TV+, Netflix (requires top tier subscription), Top Gun Maverick on Paramount+, most new movies on MAX (aka HBO streaming service).  You can also try downloading these trailers and playing them from a USB stick connected to the TV:

https://www.demolandia.net/cinema/dolby-demo-trailers-hd/page-9.html

I found an atmos test option somewhere in the Amazon Firestick app.

Forum can’t find the post so you will have to go looking for the option.

Settings > Display & Sounds > Audio & Video Diagnostics.