There's no "knowing" about it--the injectors continuously inject fuel. As Jonathan said, they're not timed. The Bosch Continuous Injection System is a basically mechanical system that sprays continuously while the engine is running. Later fuel injection systems became electronic, and they are timed to specific cylinders.

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1988 California version 260E (W124)
Anthracite Grey/Palomino
Owned since new and still going strong and smooth
MBCA member

Past Mercedes-Benz:
1986 190E Baby Benz
1967 230 Inherited from mom when she downsized
1959 220S Introduced me to the joys of keepin' 'em goin'

There are only 10 kinds of people in the world--those who understand binary and those who don't