From a collector’s perspective, and from what I’ve read, is that the late 80s is when blue line scans really became more widely used. I think the first example was Dark Knight, as mentioned above. Keep in mind, though, some artists liked sketching in blue pencil, which can look very similar. Sal Buscema did a lot (most?) of his work with a blue pencil. Same with Alan Davis, and even some more modern artists like Frank Quitely.
Sometimes, the blue pencil lines are neat, and can look like a scan. If so, you just look closely, and see if it’s slightly waxy, like a crayon. If it is, it’s pencil, and not a scan. In Quitely’s case, he starts out with really sketchy blue lines and finds his final lines from there, so it’s obviously pencil work. The blue lines don’t scan when they turn the OA into comics.
I’m not versed at all on the digital switch.