What can you tell about it's touch and sound?
Sound is often easy to compare with digital pianos, but I sometimes feel talking about touch quickly takes you to territory inhabited by audiophiles and their thousand dollar gold-plated cables. Because, in the end, beyond mechanical aspects that you can actually test, it comes down to feeling.
For touch, I only care about weighted keys and the ability of the keyboard to reliably handle 8-10 levels of touch response and communicate the same in terms of midi velocities. And this isn't something you can necessarily test in a store. Capturing the midi messages on a computer is a much better way to test this. So you have to take a leap of faith.
Pianists might care about other things as well. But I am not one.