Is there any general rule of thumb for how much force control surfaces are taking during flight given a number of G-forces, aircraft weight, and surface area?

Google for "control surface load calculator" or look up the links in the sticky threads at the top of this forum. There's a few online servo loading calculators out there. You need to carefully measure and fill in the details but it tells you what the load is at the control rod hole in the servo.

Hint... once you find that number pick a servo that outputs at least twice as much torque. You don't want to load the servo to anywhere near the stall point. Doing so or even getting close raises the current consumption of the motor to high levels and can lead to premature motor or driver board failure. Doing the twice as much deal is what is called a "safety factor" in engineering terms. Twice is at least as much as you want to use.