Push hands should be as natural as a conversion. When we are caught up with over analysing what we are saying in a social situation our speech will become muddled and whatever we are trying to communicate will seem confusing. However if we just listen intently to what the person is saying we will respond naturally.
The way of push hands (sensing hands) is similar in that we must listen intently and follow the person.
Sensing hands is a physical and internal conversation between two people.
The receiver listens and responds appropriately.
By joining, listening and following the opponents movement / energy, you will follow the natural movement of the person and be in synchronicity.
Then if the person breaks the natural flow of movement with unnatural movements e.g. a strike, it will be easily sensed.
Depending on the experience of the listener will depend upon where you can catch the change in movement of the person.
Often in tai chi chuan the metaphor of an action is like a person drawing a bow. An experienced tai chi chuan practitioner can catch the motion when the archer has thought about shooting an arrow. An inexperienced tai chi chuan practitioner may only be able to catch the the archer when he / she is pulling the arrow back.