Not a very good indicator. The voltage can be 1.3V for e.g, but the battery may not be able to drive anything because either it has not enough current, or the internal resistance is too high. It's never accurate to measure a battery's voltage without a load applied (as in the case when it's in your camera). Commerical batteries apply a known load across the battery terminals before giving you a reading.
That I am not too sure. If you want to simulate a digital camera, you will need a load with lower resistance (so it will draw more current). If you want to simulate a lower power device, e.g. a clock, you can use a smaller load. Maybe can work out from a typical digital camera's current consumption.