Another reason proving this is hard is that some artists do have a great ability to reproduce reality (this was really the most common way to measure an artist's ability before photography came along), but the economy of an artist's technique may reproduce some aspects of reality exactly, while other aspects are just suggested or completely ignored, so you'd need complete consistency.

One more reason is that simply seeing a camera obscura image or a similar 2d image may change an artist's technique, they may become aware of things they hadn't considered before (this was the case with linear perspective, look up Brunelleschi's experiment - though nothing as elaborate as a camera obscura). For a camera obscura projection, lighting conditions are very specific, that alone might shift an artist's (mental) focus from being 100% descriptive to being very vague about contours. Basically, you can't "unsee it".

Dutch artists from Vermeer's era are perfect candidates to have used lenses, because they had the technology. If you can find undeniable proof anywhere, it's probably there. Really the more proof adds up, the more ridiculous the naysayers appear. Some natural selection would do this field some good.

It was a tiny city back then, the men very likely knew each other, so Vermeer would have had access to the best lenses and the world's leading expert of that time.

Basic clue: if the painter seems more interested in showing off his command of perspective details (e.g. the lettering on someone's desktop globe, or intricate floor patterns) than, say, people, then he's quite probably using the technique.

He also points out that Caravaggio's greatest paintings (among many examples) are a combination of astonishingly deft lighting and detail combined with gross errors that you'd get if (essentially) copying and pasting bits of photographs onto a canvas. (And, hey, I love Caravaggio...) The critics would have you believe that Caravaggio was a godlike draftsman who cold realistically paint the veins on people's eyelids but couldn't put their arms in the right position with respect to their bodies.

These guys invented (analog) Photoshop in the 17th century and it was one of their trade secrets. Apparently art critics find this insufficiently awesome, or the wrong kind of awesome. (Think: science awesome vs. romantic artiste awesome.)

Hockney also traces the passing of the secret technique by putting the works of various artists on an up-down (geographic) and left-right (time) chart and can show how two artists (one of whom had godlike drafting abilities and the other of whom was merely competent) crossed paths and then suddenly the lesser artist became godlike.

The argument that Hockney is just jealous of the talent of the old masters is equally ridiculous when he shows his own work with and without the techniques. One could level the charges that the art "authorities" that disbelieve him are simply jealous of their own utter lack of talent.

tl;dr -- most "art experts" are self-deluded idiots. News at 11.