You can watch for any visual indication at all that your partner is about to move or you can wait until you are sure of what particular attack is coming, depending on your particular level. Reaction to touch is not practiced so often in most Karate dojo. In some other martial arts, such as Aikido and Judo, this type of reaction is practiced much more often. This is simply a factor of how long we remain in contact with our partners during techniques. Karate techniques usually involve separation from the partner as soon as possible after contact is made.
Grappling arts involve longer periods of contact.
The person in front assumes a stance. The person behind touches the front partner on the arm or back. The front person then attacks forward as soon as possible. For more advanced practice, the front person could attack with the limb that was touched. Obviously, it is best to be prepared to react to any of the various types of stimuli that may occur in an encounter.