One very rare and horrible morning, B737 LHR T4 about 9 years ago with freezing rain, I wanted to see the wing for myself. I got a trolley and climbed up so I was looking down at the Leading Edge. Absolutely clean I thought- I was looking from 18 inches away. I was surprised, I expected something. Just before climbing down, I stroked the bit of wing I had been looking at. Instead of aluminium, it felt rough and icy. A layer of clear ice I couldn't even see from 18 inches away, perfectly transparent! Astonishing. That very morning, an Australian 747 landed and skidded off the runway. Nobody de-iced- the airport was then closed- that freezing rain is deadly!
Always check for yourself. Take it seriously- be a pain in the arse. Get the G/E to bring you a trolley if you have any doubt or anyone persuades you de-icing is not required. They used to whinge if I opened the emergency exit to jump out and see for myself- on more than one occasion I made it clear either I had a feel for myself or someone else took it!