Just keep in mind the basics:
Range. Typically the octave is the limit as anyone is guaranteed to be able to play it, but that hasn't stopped people from writting with 9ths or 10ths to be played with one hand.
Playability. This isn't so much a 'if' question as it is a 'how easy' question regarding the piano. While it's definately possible to write passages that are just physically too wide in range to be played by one player and thus unplayable by default, determining playability based on difficulty is obviously harder because it's an individual thing. However, some standards exist, and it's always good to make your music as accessible as possible. In this respect, certain efforts should be made to write passages in a manner that can be easily sight-read, especially in large runs or other such techniques. However, not being a pianist, you may look at a passage and consider it unplayable while someone trained may look at the same passage and think "easy", so get players to review your work.
Musical tools. Don't forget the results of using such pianistic effects as octave doubling, thick chords, very fast passagework, double/triple/quadruple trills/tremolo (tremoli?), etc... For scores that really speak to the keyboardists hands there are really too many to list, and even trying to do so is pretty futile. Still, to get an understanding of how the keyboardist thinks check Bach, Mozart, Beethoven, Chopin, Debussy, Lizst, Schumann, Alkan, Rachmaninov, Scriabin, etc...
Other than that just read scores and maybe a piano technique book.