You can always ask your stoker to tell you what gear you're in... although I guess that's what you inferred by "the communication thing".
Seriously, there are quite a few "options" for gear position indicators beyond asking your stoker. You can:
1. If you've ever raced you probably learned how to check for riders on your rear wheel without getting out of your racing tuck by looking under your left or right armpit; just ride in your drops, look under your right armpit and you'll see your entire drivetrain.
2. Use bar-end shifters which give you a visual cue as to which gear(s) you are in.
3. Use STI with a FlightDeck computer or Ergo with Ergo Brain computer which display your gear positions on the "dash".
4. Use STI with Shimano's mechanical, in-line gear position indicators -- a clear plastic sleeve that your derailleur cables & housing run though that uses an orange bead fastened to the cable to indicate your gear position. Very cheap, but quite effective. Available from www.PrecisionTandems.com
and a few other places.
5. Make your own gear position indicators: http://www.thetandemlink.com/LGood/articles/gpis.html
Or, as you indicated, you can talk to your stoker and ask her to check. If you ever have kids and end up with a triplet it gets even more entertaining since you can teach your child to tell you if the chain is in baby gear, mommy gear or daddy gear. No, no... not us. We discovered tandems well after our kids were too big and too cool to ride bikes.
Last edited by TandemGeek; 07-07-05 at 05:58 AM.