In rec.autos.tech Lucia Gallo wrote:
On Mon, 21 Sep 2015 12:57:34 +0000 (UTC), Ewald Böhm wrote:

My question is HOW did the car *know* it was being *tested* for emissions?

Staying on topic, this article says the cheat only worked when there was a
DYNO involved!
http://www.chron.com/business/energy...on-6520088.php

Here are the contiguous quotes:
(begin quote)
VW used secret software ? an algorithm that detects when cars are being
tested on treadmill-like devices called dynamometers, and stealthily
switches the engines to a cleaner mode.

Because *smog tests are almost always done on dynamometers*, VW got away
with the scheme for seven years, until the "clean transportation" advocates
went to West Virginia University, which tests emissions using equipment
that fits in car trunks.
(end quote)

I have seen a suggestion that the onboard computer takes note of the
fact that the rear wheels are rotating and the front wheels are
stationary. That seems plausible to me.