It is important to develop a good ear. Try learning tunes from a recording instead of a fakebook. If you are not sure what the quality of the chords are, but can tell what the root is, try the following: 1) Listen to some recordings of tunes you know and focus on hearing what the chordal comper is doing. Learn to recognize a chord progression you know. Do this with any recording. 2) If you can loop a chord with A-B function on a CD player it helps to isolate one chord. You can do this without it, but you have to wait longer to get to the same spot and the voicing might be different. In thumb position (in the piano's range) play a note and test it with the chord. For example, if you know it is a G chord, play a Bb and see if it is OK with the recording. Do this with the 3rd, 5th, 7th and upper partials. 3) Analyze tunes for chord/scale relationships. 4) Ask a piano and guitar friend to play chords for you. Record them playing comping on a tune you know and learn to recognize the chords by ear. 5) Put on a CD of tunes you don't know and just play along with it. Keep a walking line going (even if you are wrong) and let your ears guide you to notes that are good. 6) Sing roots along with a recording. Sing other chord tones and arpeggios with a recording.
Playing some piano helps. Dan Haerle has some good piano voicing books. His and other people's voicing books are available from Aebersold. www.jazzbooks.com