They do. By simulation. Then by experimental qualification (prototype die testing), and finally by measuring packaged parts.
There are tools out there, probably mostly insanely closed-source, sign-ten-NDAs-and-deposit-$10k-please-and-we'll-send-you-just-the-brochure sorts of things, but they do indeed conduct true semiconductor physics simulations (and probably multiphysics, where relevant -- some power and RF applications including thermal gradients or magnetic fields). They can measure and model, not only the in-circuit terminal characteristics, but the properties at every point within the device, including current density, carrier density, power density, charge balance, electric field, etc.
As for what the board-level designer has... that's pretty much SPICE.
If you've poked around much in a SPICE simulator, you'll see weird constants like gate oxide thickness (TOX), channel length, width, junction area, etc., which go into some of the models, and do indeed correspond to certain attributes of geometries that were typical at the time. (We're talking 70s-80s tech here. Ancient stuff!) None of the quantities are *quite* what their names suggest (especially the geometry-dependent ones, like the 'channel sidewall' stuff, which really just serve as a corrective hack to tweak expected geometries), but they're close enough to count as "physics based models", at least for the given subset of "physics" they relate to.
Tim