Newbie question: how to tell if the battery is in good shape?

Good question. In meantime, might want to try google search here with 'hybrid battery diagnostics' or checkup.

 

I don't believe there a really great surefire way to judge HV pack condition of one that hasn't failed. I have seen Predictive battery failure analysis for the Prius Hybrid, for example. I am curious to know if there's a better way...

As for 12 volt, http://priuschat.com/forums/newbie-forum/73400-weird-stuff-happening-mpgs-dropping-test-battery.html is a good method but you should let the car sit overnight before doing this. Don't have it taken to a Toyota dealer to have them "test" it. Even the repair experts here (e.g. Patrick Wong) find that the tests they do seem useless and sometimes give incorrect results in either direction.

 

Skip the dealer. The inspection that you pay for is just as worthless at the 147 point inspection that you get with an oil change. Very, very few "certified hybrid technicians" know what to look for to test the hybrid battery, and one of the best tests, letting the car sit for a week and looking for high self discharge, is completely impractical. The main things to address are: Does the engine have clean oil in it? Are the cooling systems full? Does the engine water pump leak? Has the inverter cooling pump recall been performed? Everything else, you will be able to feel it when you drive the car.

Buy yourself one of these:
http://www.amazon.com/dp/product/B000EVWDU0?tag=priuschatcom-20 the prospective test drive, insert your battery tester to the power port. With foot off pedal, doors closed, press the Power button once to go into ACC mode. Voltage should be 12.x here. Turn on the headlights for 10s to burn off battery surface charge. Battery should not go lower than high 11.x. Press the Power button again, foot still off brake, to go Ig-On. You should still have high 11.x. If you see a steady 10.x or less, the 12V is bad.

Foot on the brake now, press Power to make the car Ready. Let the car warm up to the point of idle shutoff. Reset the MPG meter on the consumption page and go for a >10 mile round trip test drive, with city+highway driving. If the MPG is less than 40, or the car never shuts off, then also suspect problems with the 12V or hybrid battery.