Most all sealed bulds filament will increase resistance as it heats up as most are made with tungsten, when off the bulb is a short. Positive coefficient. This is why tungsten is used for some oscillators.
Accurate measure can only be determined at the batteries max current drain capability, sometime this can be in the range of 10's of amps with good nicads C size or a few amps for AA size, a several hundered mA for alkalines AA size like close to 1amp but not practical since the hotter alkalines get the less current that it can drive same way if they get to cold, alkalines are very temp sensitive, and can blow up if driven like a nicad. However tests using a low power load would yield capacity to low level loads and most times this would yield a high MaH rating simply because less of the batteries chemistery is being wasted to heat.
Quality batteries have been made in such a way that they can drive heavy loads (20-30 amps C2sce, scrc sub C class and 5 amps AAi sub AAsae, sarc class typical) and are tested to perform against these loads to show the MaH rating in the end, in fact a battery in a high current sub class can perform low level loads and actually increase propotionally to MaH produced.