I watch for the light to go out and then wait for the buzzer to stop before starting. I've heard two things about this though,
-People on here largely agree the glow plugs stop glowing as soon as the engine cranks.
-indy Mercedes mechanic I'm friends with says the glow plugs should stay on.

I normally just wait for the glow plugs to "click." so I know they're done heating. I havent checked the rack dampener yet.