Drivers are usually very good about using their turn signals. This is yet another way that a professional driver stands out from the rest. Four way flashers are often used to communicate to others that a truck is moving very slowly, when parked along the side of the road or when backing. Drivers are usually very good about using the proper signals, turning or 4 ways, when needed. Again, it is a sign of professionalism.
When backing out of a truckstop spot that is between two other trucks and in effect, completely blind, drivers will turn on the 4 ways and the CB radio and proceed slowly. If there is someone coming or some other obstacle, most the time someone will say something on the CB radio to you. Be sure you're also watching for other trucks' four way flashers when driving through the truckstop lot because it is usually going to mean that that truck is preparing to back out of his/her spot. As a courtesy, stop and allow him/her to get out of the spot. By you sitting there waiting, you'll also be helping block the space.