Final ISO when using Flash
Most will just snap a shot and look at the image on the back of their super-fancy camera and see it it looks bright enough.
Once they get seriously advanced, and they are paying a lot of attention to their photos, they might shoot a test frame, pop the card from the camera, insert it in a computer, open Photoshop, and look at a big version.
... and check the histogram, which is a good and cheap way of asserting your exposure.
If you don't know how, search the forums.