How do I tell when the batteries are fully charged?

I bought some AA NiMh batteries to run my Quark AA2 flashlight. I put 4 of them in my charger and after about an hour I tested the voltage and they were at 1.28 or a little higher. I put them in the flashlight and they ran out of power in about 15 minutes where they were supposed to last for 1.3 hours. I read the instructions on the charger and it says 17 hours for AA NiMh batteries so I charged them for about that long. I then put a couple in my flashlight and it ran for 1 hour and 10 minutes. My question is this. How do I tell when the batteries are fully charged? I assumed that if they said 1.2 or higher on the meter, they were fully charged. Guess I was wrong.

NiMH batteries are more like 1.4 volt when charged, but just measuring voltage is not a very good way to tell if they are full, you need to apply a load while measuring voltage (Battery testers work this way).
You can see an example in the DMM guide I have linked to below.

Chidwack said:

I read the instructions on the charger and it says 17 hours for AA NiMh batteries

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It seems this is a simple timed charger.

My question is this. How do I tell when the batteries are fully charged?

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You can work this out from the charger specifications and the capacity of the batteries.

Suppose for example your charger supplies 150 mA (look at the rating plate on the charger). Further suppose you have AA batteries with a capacity of 2000 mAh (again, look at the rating on the battery). If your batteries start out empty, what you need to do is divide the charger current into the battery capacity and add about 20%. So for example, 2000 mAh / 150 mA = 13 hours. Add on another 20% of this gives 16 hours. So that is your estimated charge time. In short hand, the calculation is 2000 x 1.2 / 150 = 16. You will have to change the 2000 and the 150 to the values from your actual charger and batteries.

Even load based testers fail at measuring nimh most of the time. At best you can tell between full, almost empty, and empty, but even that varies between the particular cell and its age,.

While NiMH cells are referred to as having a voltage of 1.2V, this is the 'nominal' voltage, and not the peak voltage that the cell achieves when it comes off of the charger. Much as LiCo and LiMn cells are quoted as having a nominal voltage of 3.6 or 3.7V but are charged to 4.20V (or in some specific cases, 4.35V), so with the NiMH cells, the voltage is higher when freshly charged, as HKJ stated.

1.44 or 1.45V seems a fairly common peak voltage for NiMH cells, but depending on the exact cell/charger combination, can be as high as 1.5V.