7-zip files on Linux with AES-256 encryption

7-Zip will refuse to encrypt the data unless you provide a password, since it won't know what key you want to encrypt it with:

7za a myfile.zip test.txt -tzip -mem=AES256 -mx9 -pmypassword

How can I (and more importantly the end user) be sure that its using AES-256 as the encryption method?

You can use 7z l -slt myfile.zip, it should output AES-256 (amongst other things).

If you choose the 7z format (e.g., with 7za a -t7z myfile.7z test.txt -tzip -mem=AES256 -mx9 -pmypassword), 7z l -slt myfile.7z should output 7zAES:19 (which is the same as AES-256).

https://sourceforge.net/p/p7zip/patches/25/#3da5 :

Igor Pavlov: 7zAES:19 means AES-256 + (2^19) SHA-256 iterations in password to key function.