There's a FREE benchmark test app in the store that has a battery test option. The app is called TabletMark v2. I just downloaded it >900mb and ran it for 2 iterations which takes approximately 40 min per iteration. If you select the Battery Test option the iteration count is disabled which I assume means its going to run until the battery dies.
In order for any comparisons to be valid everyone would need to run with the same configuration. i.e. auto brightness disabled, airplane mode, Bluetooth off, brightness at 50%, Sound at 50%, no USB connections, keyboard disconnected. Need to ensure same set of startup apps and nothing running in the background (details TBD). I'd say as prep run the device down to 20% battery then charge it to 100% then shutdown for 10 min, startup, launch the test app and set a configuration. Them disconnect the power and start the test.
Here's where it gets trickier... especially since I haven't done this yet... it appears the test will record a battery score automatically [I hope
] this could take hours and hours which many just don't have.
Alternatively if say you just ran 6 iterations which should take around 4 hours then recorded the % battery life there's a risk of error but might be a shorter option. in addition if you miss the end then your manually collected results would be off by some amount.
Both options would allow people doing the same test scenarios to compare their results. I figure results within perhaps 10% are normal and if your results deviate by more then it bears further investigation.
Clearly other things they may be installed could have adverse impacts. Such a the DisplayLink driver I have installed I have noticed uses a lot of resources and the USB dock does some weird stuff waking up whatever is attached periodically. not sure if those drivers go completely silent if its disconnected.
I'll experiment a bit more with this and let ya know how it goes.