Erm. Let’s see.
By breadcrumb trail, would probably be the most accurate answer. Seeing where other people have published and knowing their work. And liking the look of the place when you get there. And thinking you have something that might work for that particular place.
I get Duotrope’s weekly email and like surfing through that for new possibilities. I also have staple markets I’ve been trying for with no luck so far — Stirring, Stickman Review, Juked, Ghoti, Kaleidowhirl, to name but a few. Places I’ll always think of first when I have a new piece that might - just might -be a fit.
It’s not like one always has endless pieces of work available to go out at any one time. Although I do like to keep everything I feel is ready (what’s that?) out there working, and those pieces usually get marched out again pretty smartly after they come home rejected. After a bit of TLC, if they’re lucky.
So, yeah. Scientific. Breadcrumb trail.