How to tell if dmg file contains universal binaries, i.e. if it has been compiled for arm64 hardware
Solution 1:
A DMG is just a container, it doesn't know about it contents.
If application is distributed without a package (so it's application you can just drag&drop to /Applications) you can run
hdiutil attach THE_DMG
file /Volume/THE_DMG/THE_APPLICATION.app/Contents/MacOS/*
to see what kind of binary it is.
Solution 2:
you can get info on the app, by right click and get info, or cmd+i, and see what it is designed for. Intel, Apple Silicon, or Universal.