From no-tech to low-tech and beyond, here is what I have come across:
1. Captain has super memory and recalls exactly which chainring and which cassette or freewheel sprocket is in use at any time, thus knowing how many cogs are left to shift to if down-shifting or up-shifting is in order.
2. Captain is humble enough to ask stoker what combinations are in use, and stoker can at least see and say "there are two bigger sprockets left to use".
3. In-line indicators on the shift cables -- either the model aircraft collars with their mini-set screws, and stripes or other alignment devices on the downtube; or, the Shimano in-line tubes with the little red donuts on the cable that go back and forth as the cables move.
4. Tiagra ST-4500 brifters with visual indicators on the tops.
5. Campagnolo Ergo-brain, or Shimano Flight Deck, electronics.
Leaving aside any issues of brifter shifting systems (ShimaNO versus Campy versus SRAM etc.), has anyone used the Tiagras (for single or tandem bikes), and what was your impression?
And, other techniques (in addition to the "my legs tell me when to shift" answer) would be appreciated.