On Canon ETTL and ETTL II flash systems built into Canon's EX range of Speedlites, a pre-flash is fired just before the main flash in order to obtain the correct exposure. Because the pre-flash occurs immediately before the main flash, it is very difficult to identify the two separate flashes and it looks as if just a single flash has been fired.

However, to make sure that the Speedlite is firing a pre-flash you can set the Speedlite to Second Curtain Sync and the shutter speed on the camera to two seconds. This will create a gap of two seconds between the pre-flash and the main flash, allowing the two separate flashes to be easily identified.