There are a few tools to measure how much power drainage there really is on your Windows Mobile device. One of them is a today screen plugin called HomeScreen++ (previously known as BatteryStatus). Although this is a very handy plugin to measure actual power consumption, the disadvantage is that it contains a lot of other functions we don't necessarily need. Also, it is active all the time, you can't pause it except by disabling the whole plugin.
Here's an alternative program that might just do what we want: acbTaskMan by acbPocketSoft is a nifty free program that not only shows batteryconsumption and cpu usage, it actually makes a nice graph of it. There is a paid version too with extra functions.
Here's a list of features taken from their website: More...