You can look for a PTC (Pending Trouble Code) when the light is off, it's like a code that is on the verge of throwing a Check Engine Light. When it grows up to be a DTC (Diagnostic Trouble Code), your CEL will illuminate. If the CEL goes out, I'm pretty sure that the DTC that set it will be gone, too. Some codes will go away if the computer sees a certain amount of drive cycles or key on events without seeing the same problem again. A CEL doesn't necessarily mean that something is wrong right now, but may mean something happened that worries the computer and it wants you to give it some attention.
Depending on the type of code reader and the skill of its operator, you may be able to look up what is called a "Freeze Frame". When a DTC is set, the computer stores the current sensor conditions at the time the code is set (like RPM, Vehicle Speed, Coolant Temp, O2 readings, etc.). So you can see what was going on when the code went on. But this goes away after some set period as well.
Best bet: next time the Check Engine light comes on, get over to an Autozone and you can get the code read for free. Without the code, there is no point in trying to diagnose or fix the problem because it could be hundreds of things.
