Hey Locke,
Marc had trouble explaining how you'd be able to tell which side would be brighter. Before showing you my take, short disclaimer: I have no further source but my own interpretation.
That being said, the general principle seems pretty easy at first glance: Take a look at the blue square, which represents the simplest case: Top down view.
It is very easy to say which side gets more light, but since we need something that translates well in perspective, I've drawn in the 45° degree angle which is very easy to find once you have a square in perspective. It's also quite convenient since if the light would hit at an angle at exactly 45° the lines would match and both sides would be equally lit.
Now it's just a matter of translating that to perspective, but the usual rules apply so nothing new here. The only tough part is finding a square in perspective, There's a couple of ways to construct those or you can just guess. For the guessing part it is best to chose a square in the middle of picture plan, or otherwise you'll have to fight heavy distortion and subsequently will be miles off, just like me. Just pretend those squares I drew in are perfect, it doesn't change the outcome in this case but makes it easier to see.
For chosing between the left and the front I'd draw in the purple square and just compare it to blue. Just a slight deviation in this case, but you'd see that the front side gets more light.
For chosing between top and the front plane I'd draw in the orange square, again comparing it to blue. You now see that the top gets more light, as it's on the opposite site of the 45° line than on the blue square.
Following that logic, the top is lighter than the front which is lighter than the left side. I think it's quite easy if you really wanted to check.
Do keep in mind in reality, that even within the plane there would be a value shift from light to dark as the plane extends away from the light source, so while you'd be able to compare the planes right at the edge pretty accurately, you wouldn't be able to compare points that are further away from the edge. There are also differences between light sources and how they behave in different atmospheres.
Still, I think all this is more of an interesting thought experiment than actually useful. It's way harder to spot mistakes than if for example the perspective was wrong. And since light is a tool to convey form, one objective as an artist is to chose your lightsource in a way that your forms read so there's rarely any room for ambiguity.
And since you mentioned Istebrak to me, I really liked how she thinks about this. Imagining an arrow that's pointing straight away from the surface, the more directly it's pointing at the lightsource the more light that surface gets.
