As previously stated, you have to be within 1 second of a car in front of you....any car, the one you battling for position with, a lapped car or a car lapping you.....when you hit the DRS detection zone.
As there's no split stopwatch for the detection zone, like for sector timings, you have to estimate what your distance is. You also have to know where the DRS activation and detection zones are. So far, the only track I've seen with an actual notification is the German GP, even though the CM forums says there's marker boards stating it at every track. Once you figure out where the actual activation zone, you can look before it for a white stripe across thet track to find out where the detection zone begins.
I use my installation lap to try to ID that.
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"Sometimes I wonder whether the world is being run by smart people who are putting us on, or by imbeciles who really mean it."
Mark Twain