How do I tell at a glance whether a Warjack is focus hungry or not?
It may not be easy as a new player since some of that will come with experience.

Ask what role the warjack is going to fill and then how much focus it will normally need to do that.  Will you need to spend focus to run/charge/power attack?  Will you need to boost to hit? To damage? To make extra attacks?  Do they have something that lets them save on focus (this is why the Berserker is focus-light)?

A basic jack designed to run into melee and whack something is going to need more focus than a 1 ROF ranged jack with a good RAT, or one that is mostly just used for its arc node.