No indicator has been developed that approaches the sensitivity and effectiveness of telltales along the luff of the jib. These yarns or ticklers monitor the flow of wind across the sail. Telltales are only an aid when the sail has wind flow across both sides, i.e., when sailing angles between beating and beam reaching. When sailing lower than a beam reach, the sail is catching wind instead of working like an airfoil.
Telltales come in pairs, one on each side of the jib.
The best set up is to have three sets evenly spaced along the luff so that you can see how the wind is flowing at different heights of the sail. Telltales should be anywhere from six inches to 18 inches aft of the luff, depending on the size of the boat the bigger the sail, the further aft the telltales should be. Also, the farther forward they are, the more sensitive they are. All UK Sailmakers sell inexpensive packets of green and red telltales that stick on to your sail. When applying telltales, always put the starboard one of each pair higher than the port one.