Is there a way to get a list of all classes from a .dex file?

Use the command line tool dexdump from the Android-SDK. It’s in $ANDROID_HOME/build-tools/<some_version>/dexdump. It prints a lot more info than you probably want. I didn’t find a way to make dexdump less verbose, but

dexdump classes.dex | grep 'Class descriptor'

should work.