What's the shelf life of fuel and how long will it stay good?
Glow fuel will last almost indefinitely if it is protected from contact with air as much as is practical. Most often, fuel is "killed" by absorption of moisture from the air. Unlike gasoline, mehanol — the primary ingredient in fuel — is hydroscopic and absorbs moisture aggressively. A container of fuel left slightly open for an hour in humid weather will be pretty well ruined.

How can I know if my fuel has absorbed too much moisture and is there a simple test I can perform?
The "simple test" is to run the fuel. The first sign of moisture absorption is an inability to obtain a normal idle — or any idle at all. Typically, the engine will be difficult to start and will die, or it will begin to run roughly as soon as you remove the glow battery. To verify, drain the tank, fill with fresh fuel, and restart. If the engine runs smoothly, then there's little doubt that moisture's the problem.

Why doesn't someone invent a pill that can be dropped into fuel to absorb all the moisture?
Not possible. Methanol is 100% soluble in water, and short of an expensive distillation process, there's no way to "un-absorb" the moisture.

What can I do to protect my fuel?
Keep fuel tightly sealed and minimize air space in the container. Open only when necessary and keep all tank vents sealed (hook the filler and pressure lines together after each flight with a short piece of tubing). Plastic keeps fuel better and longer than metal, and the containers don't rust. If you do use a metal can, replace it from time to time to prevent corrosion.