Let’s shout together: “HOW CAN YOU KNOW IF ANY KIND OF SNOW IS ‘ADHERING’ TO THE SURFACES OF YOUR AIRPLANE, AND NO ICE IS BENEATH IT, IF YOU DON'T REMOVE IT?”
“No clean aircraft? No Fly!”
Most US operators require a close visual inspection and/or tactile check be performed if any contamination is observed on the wing surface (whether it's snow or ice).
FAA 8900.1 Volume 3, Chapter 27, Section 2 pertains to the process by which ground de-icing programs are approved for air carriers under parts 121, 125, and 135.
For scheduled air carriers, by regulatory requirement of
FAR 121.629, a manner by which the operator can determine whether or not contamination is "adhering" to the wing must be described in detail. All flight crew members and de-icing technicians and vendors must be trained on the recognition and removal of ice and other contamination on the critical aircraft surfaces. If an air carrier lacks these procedures they cannot operate in ground icing conditions.
The common practice is a close visual inspection of areas suspected of having contamination and often a tactile inspection to determine whether or not de-icing is required and after the de-icing process to determine that the contamination has been adequately removed. In short, in the US all snow must be removed from the wing either by mechanical means (guys with brooms), by putting the aircraft in s heated hangar and letting the snow melt off, or if the temperature is such that ice will not accrue - using heated water to remove the snow. Of course using the glycol/water mixture is also an option though airlines try to avoid it due to the cost.