Well all departures have a Scheduled Time of Departure (STD), which obviously the operating crew will know, but also the ground handling agent; who provide the pushback tug and ground crew. They usually turn up with anywhere between 15 and 5 minutes before pushback, depending on how busy they are, and connect a headset to the aircraft in order to communicate with us in the flightdeck. Failing that you can revert to standard hand signals in order to communicate with the ground crew before, during and after the pushback.
Hope this helps
Atreyu
