There are some useful tells. My favorite has the been the classic quotemine, where creationists quote one sentence of Darwin’s — “To suppose that the eye, with all its inimitable contrivances for adjusting the focus to different distances, for admitting different amounts of light, and for the correction of spherical and chromatic aberration, could have been formed by natural selection, seems, I freely confess, absurd in the highest possible degree” — to claim that Darwin was stumped by the evolution of the eye. As everyone who has read the Origin knows, what he was doing there was setting up a rhetorical question, which he then followed by three pages of detailed description of exactly how such an eye could have evolved. When you hear some creationist say “absurd in the highest possible degree,” you know right away that they haven’t read the book.
There’s another great example, though, that’s an even better demonstration of your opponent’s illiteracy. That is when someone cites The Selfish Gene and then goes on to rail against the horrors of evilution and the way it encourages people to be righteous bastards who kill and steal and rape their way to dominance. They haven’t read the book! All they’ve done is scanned a three word title and leapt to a series of absurd conclusions! (Yeah, Mary Midgley, I’m givin’ you the squinky eye.)
Ken MacLeod exposes the inanity of this claim in some detail. It really is astoundingly common for people to expound on how Richard Dawkins was arguing for the rightness of Thatcherism or whatever reactionary conservative policy they think he was endorsing, and get the whole story completely wrong — it really is a great tell. Unfortunately, it seems to expose left wing idiocy more than that of the right, but only because I think the righties make the same invalid assumptions, but since they like that error, they tend not to criticize.