It is indeed a pity that this isn't listed anywhere in Settings, not even in Developer Options. The minimum-change way to get the information is as follows:
Turn on Developer Options and USB debugging
This is familiar, and this answer isn't the place to reiterate that.
Ask Android's Linux kernel
adb shell uname -m
The reply will tell you what architecture your Linux kernel is running:
- "aarch64" is 64-bit ARM; that's the only kind of Android I have running.