VCDS or some other scan tool, or the hidden menu on the climate control apparently, but I've never used that.
VCDS takes the reading directly from the temp sensor, which is fitted into the main thermostat housing.
The Dash gauge takes the same reading, but manipulates it as described above. 80 - 100 has been fudged to always be 90 on the gauge.
At this time of year when cruising on the motorway, the water temp will be in the mid 80s with working stats.