Long touch has become a common input action on touch devices. Does anyone know of any sign or symbol that means "this only responds to long touch?" On all the long touch uses I've seen you have to learn that the only way to accomplish some things is via a long touch, so some kind of visual cue that something (button like thing) is long touch activated would facilitate the learning of the app.
I don't know of such a symbol, however, you could use an animation of a process to emphasize the duration required for the click.
Example 1: a button that is shaped like a screw head that screws into place while you hold your finger down on it, until it is in place (and if you let go beforehand, it screws out by itself).
Example 2: a lever that moves slowly into position when you hold your finger down over it.
Mobile Tuxedo has an interesting array of options for touch gesture icons, and their solution is to couple it with a timer icon (4th column in their matrix). If there's a specific duration of hold you require, or it changes by control, I think placing the minimum duration in seconds along with or in place of the timer would work.