Eye Drive

The Eye Drive was a device used by the Silence to allow selected individuals, such as Madame Kovarian, to remember the memory-proof Silents running the order. An Eye Drive looked like a silver eye patch and was worn over a person's eye. The device formed a neural link with the wearer's mind and continually reminded them of the existence of the Silents.

The Silence built a fail-safe into the devices that caused pain or death when activated, depending on the individual. This fail-safe was integral to the functioning of the device; any duplicate included it. (DW: The Wedding of River Song)

Madame Kovarian was always seen wearing this device. Until the events of Area 52, the function of the device was unknown. (DW: Day of the Moon, The Curse of the Black Spot, The Rebel Flesh / The Almost People, A Good Man Goes to War, Closing Time, The Wedding of River Song)

After examining Madame Kovarian's eye-drive in an alternate time line, Amy Pond and her army created their own versions. These were worn by all of them, including the Eleventh Doctor. Kovarian's drive seemed to be a colour slightly different from the duplicates, most probably because hers was made by the Silence, not Amy Pond's "army". Unfortunately, the soldiers were unaware of the built-in fail safe in the design. When the Silents broke out of confinement, they triggered the fail safes, killing many of the soldiers. (DW: The Wedding of River Song)

So far this is the only method to be continually aware of the Silence.