[jonnythan]

Well, you can, just not very easily. One of the easier ways would be to turn the phone's radios off and disable all background services, set it to a certain brightness, let it fully charge, then play a video in a loop until it dies. Repeat this 3 times for each battery and measure the time.

Another way would be with a special battery charger that has a discharging function. There are some fairly fancy universal li-ion battery chargers that will charge your battery and then do a full discharge, complete with measured mAh rating (it calculates this based on current draw and time).

Of course, neither of these tests will exactly correlate to real-world usage but they can potentially give you a good idea of how various batteries stack up against each other.