Wool tufts
My outfit had them installed onto the inboard top of the wing so that we could inspect them through the cabin windows which were especially handy for clear ice.
I was sent forward whilst dead heading by my skipper between GVA and ZRH on a North African carrier as he had spotted a sheet of clear ice caused by cold soaked fuel with relatively high humidity and temperatures in double figures; it had been missed by our engineers.
We had a German skipper imprisoned in Anchorage after he climbed onto the wing of a local carrier after the crew refused to de-ice the aircraft that was covered in dry snow.
My next trip there I witnessed one of their aircraft disappear in a cloud of snow as it rotated and the airflow change dislodged the snow on their wings. Normal ops don’t clear dry snow. Never flown with them nor will.
Only ever had one disagreement after we had passed hold over times with a skipper who was a big cheese in the military and refused to return, I re checked the wings and decided that it was time to threaten standing on the brakes time but ATC closed the airport before we were cleared to line up.