Well, that's average mileage overall, but really good mileage for Toyotas, which tend to build miles faster than other cars.
Are you buying it from a dealer? Really, all the usual stuff for test driving is all that applies.
Don't test drive in the rain (tire noise on wet pavement may drown out any funny sounds).
Drive around in a circle (slowly) but with the steering wheel just at full lock giving it a little gas - make sure the CV joints aren't clicking.
Try some hard and slow accelerating, make sure the Transmission doesn't slip.
Try the AC AND the heat (most people forget to test that, especially in the summer).
Looks like there are a few little service bulletins out there. You can find them
HERE. Ask the dealer about those.
Get a Carfax report, they're nice to have as they give you an idea of how well the vehicle was maintained. If they took it to a dealer to be serviced for instance, it should show up. If there are no service records then there's no telling who worked on it.
But, as far as specific to the Echo, I don't think you have much to worry about. I bought mine at 75000 miles and the only thing I had happen was the power lock on my side stopped engaging. I could still lock and unlock the door with the key. Fixed it under extended warranty. Been worry free for almost 4 years now!