Trial and error. Over many years of shooting, I find I like to leave my cameras in a "ready to go" state with a*fast normal-ish lens fitted. And most of the time when I grab the bag to go shooting, I leave it in place.
Whether that's a 35, 40, or 50 on 35mm format, a 21 or a 40 on APS-C, a 25 on FourThirds ... depends on what's available. I happen to like both 21 and 40 on APS-C as normal-ish, all three of those focal lengths on 35 FF, and the Summilux 25 on FourThirds is just right.
My second lens for nearly all cameras EXCEPT the Leica M9 is a portrait tele: 45 on FourThirds, 90 on APS-C, 90 on 35FF. For some reason, on the M9 what I always seem to want when I don't want the 50 is a 28.
Again, trial and error—and long habituation—rules the selection process. There are times when I get into a long lens mode (90/135/180 on 35FF and equivalents), and other times when I get into a wide mood (20/24-ish on 35FF). But those are*the exceptional moments rather than the rule.
G