So how do you know if your battery is bad ?

Unless there is a "blue screen of death" on the dashboard - you dont - all you see is that the battery doesn't last as long as it used to (...and are you sure ?)

I'm guessing that a not lot of Prius batteries around going to fail outright, they are going to lose capacity between the 10 and 20 year mark and then (maybe) fail completely around 20 years. And I'm going to guess that when the battery does fail completely:
a) the IC motor will still move the car
b) there is some kind of protective fusing that will keep the batteries from heating up/melting things/causing catastrophic damage - if they DIDNT put some kinda protection between cells, then they will have earned the fallout that will occur the first time a Prius gives poor kid a 3rd degree burn and the internet gets a hold of the pictures.

That being the case, a 10y/o Prius is still a usable machine - it'll run its IC motor more, and fuel economy will take a hit, but if you never calculate your economy or call up that screen on the display - you'll never notice it. Even then, I can't see the economy of a 1.0l engine going less than 30mpg... Most batteries fail slowly over time by losing capacity.

If the rest of the car holds up ok (remains to be seen- they do have a LOT of computers...) and they still run when you turn the key, they will hold their value decently - I'm guessing that a small hybrid with a dead battery will still get 30mpg - which is better than the 1997 VW jetta (that I just sold) got.

We'll see.

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2009 Kia Sedona
2009 Honda Odyssey EX-L
12006 Jetta Pumpe Duse
(insert Mercedes here)

Husband, Father, sometimes friend =)