I imagine that if any of the hydraulics systems is about to fail, it would be shown on the EICAS (Engine Indicating and Crew Alerting System) eventhough it is not related to the engines, there are displays on the EFIS that show if all the systems of the aircraft are working properly. In such case, the crew has to take action and declare an emergency to the nearest suitable airport. In the event of a complete pressure loss in the hydraulics system, I think what the crew still can do is get the landing gear out manually and try to control the aircraft by dealing with the power of each engine.
Remember in 1989, the crash of United 232 in Sioux City Iowa? It was a DC-10 that lost its n°2 engine which made the hydraulic line fail. The crew did all the best they could to land the DC-10 with a hydraulic failure, flying the aircraft was almost impossible so I beleive they(the pilots) dealt with the power of each of the remaining two engines to steer the aircraft. Upon landing, the back section of the passenger cabin went on fire. Out of nearly 300 passengers and crew, about 180 survived.