The granddaddy of stock market divergences is the Dow Theory.

Divergences, in case you didn't know, occur when one market moves (up or down) and another does not follow suit. In the Dow Theory case, if Dow Industrial (DJI) marches ahead (or down) and Dow Transports (DJT) fails to follow along (either up or down) then a divergence occurs suggesting that the move is unconfirmed and, therefore, is in doubt. This performance divergence suggests a weakness in the move.

While far from perfect as a predictive tool (and most definitely not one that should be relied upon solely), the divergences principle does help inform one as to the strength of a given market trend. As the accompanying chart shows, we have such a Dow Theory divergence of sorts underway. Not enough to shout "get out" but, when coupled with other indicators (previously noted) is enough to join the yellow warning signs club.