How to tell if my system is "multiarch"?

Per the LTS Enablement Stack wiki, "to install the newer HWE kernel derived from 15.10 (Wily)" there is a different set of packages to install depending on whether a system is multiarch or not.

How do I tell if my system is multiarch or not?

The wiki is "hinty", saying "you may find you need" to do this.

dpkg supports installing packages for other non-native architectures using a system called multiarch. It was introduced in Ubuntu 11.04, so any later release should have it.

You can use dpkg --print-foreign-architectures command to find the added foreign architectures on multiarch e.g.:

% dpkg --print-foreign-architectures
i386

To find the native architecture, use dpkg --print-architecture e.g.:

% dpkg --print-architecture          
amd64

Alternately, you can also read the file /var/lib/dpkg/arch to get a list of added architectures (first one is native, although you can remove it):

% cat /var/lib/dpkg/arch             
amd64
i386