Mostly just the usual things you look for in any car: maintenance records, accident records, signs of abuse, etc. There are inspection firms that can help you with this. I have both a 2001 and a 2004 Prius which have been among the best cars I've ever owned. They have 80,000 and 108,000 trouble-free miles respectively.
And the silliness about the brakes in the one answer is just that "silliness". The Prius has four braking systems: regenerative, friction, engine, and parking (not to mention brake-assist which detects if you have an emergency situation and adds additional force to the brakes, and brake-override which cancels acceleration if you press on the brakes firmly). You have to really try hard not to be able to stop a Prius.
So if the Prius you're looking at checks out, go ahead and get it. The Prius is a great mid-sized car, reliable, easy to maintain, roomy, and fun-to-drive.
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