From flyfisherman.com's blog:
Let’s talk about how to read a permits eating reactions. Why is this so important? Well your ability as a fly angler to read the permits body language during a fly presentation will help give a clue as to whether the permit has eaten the fly or not.
This photo shows you a permit in a total vertical mode. This action usually is high odds that the permit is eating your fly right now. This would be a good time to strip slowly trying to feel the line come tight
If you don’t strip to fast and the fish has not eaten the fly then the slow movement of the fly moving on the bottom will be a noninvasive movement and hopefully entice the fish to eat the fly. If you strip to fast then if the fish didn’t eat the fly you risk adding to much movement to quickly and possible spooking the fish.
Keep an eye on the permit and if you see a vertical head stand like this pucker up and get ready baby!