I have to jump on the "Service Manual" bandwagon. In my maintenance checklist, checking the spokes is done a 5K and again at 20K. When I checked them at 5,000 I found a total of 2 that needed to be tightened. I now check them during every 5000 mile service. Not very scientific, but when the bike is on the lift I rotate the wheel and give each spoke a light tap and listen for a tuning fork sound. Any spoke that sounds flat get tightened a little at a time until I hear a steady tone.