That breakout above the downtrend line is a distant early warning signal that the psychology of the markets is changing.

FORBES: The Dollar Peeks Above Its 13-Month Downtrend Line

Fiserv already uses some of its bank customers' data to spot fraud, even if it can't yet serve as an early warning signal of potentially sleazy activity.

FORBES: Global 2000

If shopkeepers ever start separating their notes, it will be an early-warning signal that confidence in the Bosnian notes is slipping.

ECONOMIST: Building Bosnia on banknotes

The money supply can often be a useful early-warning signal.

ECONOMIST: Remember monetarism? It may be coming back into fashion