DaddyJJ,
Everyone will differ on their opinion when judging a piano sound. I've heard some say they love the Yamaha pianos in PSR's, and some say they've heard better on cheaper Casio's Personally I listen for several things. I listen for a natural sound.., listening to see if the sample has noticable looping. I also listen for realistic characteristics such as string noise, hammers hitting strings. I also look for velocity switching in the sound.. A good sampled piano will have this.. A good example: Test the lower reigister. Hit a key with medium force, and then hit it hard.. When you hit it hard you should hear more of a metallic ring, as this is what happens on a real acoustic. If the keyboard is only 61 keys, I will always use the octave shift and test both the upper and lower registers to see what they sound like.. Sometimes a keyboard will sound great on the 61 keys, but when you listen to lower and upper rigisters they don't sound so good. I also listen to how long the sample is, meaning I'll hit the key with medium pressure and listen to how long it takes for it to fade out..

Squeak