There's nothing new here. Before I was into bikes I was into toy trains. The number of forgery Fairbanks Morse Trainmasters was astounding. SOme of the best experts were fooled by the counterfeits. I never had that problem because I never bought a "mint" trainmaster. The two I owned were well used along with all my F-3s and Berkshire. Same for bikes. Anything that's steel and somewhat expensive better have some rust on it or I'm walking. Because bikes are used outdoors, those tell-tail rust spots should be no trouble to find. Also, I don't mess with "custom built" unless it's a garage sale find.
I understand how this problem can happen with a toy train, but not bikes. There's just too many ways to tell.