Mostly, I think of this as it refers to people who were taught to read, but do not do so for entertainment–in spite of claiming they do. Not legitiately illiterate people, or people who simply don’t enjoy reading. I find that the former tend to hide it, while the latter openly admit to not reading for fun.
To prevent pissing off people, I’ll focus on characters instead. It’s an error or offence when a character makes an empty/untrue claim like this. A mistake on the part of the writer.
First, most common clue: they talk about reading, either frequently or passionately. Most people who really read, especially a lot, are more likely to talk frequently or passionately about specific books, or topics related to reading. For example, they might enjoy deep discussion about The Dresden Files, or can talk for hours about the differences between classic and contemporary fiction.
I’ve never heard an avid reader gush about how wonderful reading is, unless the topic came up. And by “came up”, I mean someone else said reading was a waste of time, or that the movie is always better than the book. We do all love to argue.
Another clue: the character is never actually seen reading. If some story element prevents them from indulging in the hobbie, then have the character draw on knowledge gained from previous reading. See Klaus Baudelaire of A Series of Unfortunate Events.
Oh, and please don’t have the character play storyteller. For any reason. It’s twee at best, and cringe-inducing most always.
In line with the last clue, although a bit of converse: the character is seen reading despite the fact that timing or environment should prohibit the activity. See Throne of Glass. …or rather, don’t. That book is abysmal.
If a character is involved in an event that will grant her every wish upon success, and a fate worse than death upon failure, then she would not realistically stay up all night reading the night before this event begins.
There are probably other signs, but I’m getting tired. If a character is touted as a passionate reader, yet exhibits any combination of these clues, then you have caught a writer attempting to make you sympathise with their character through the crappy means of, “s/he’s just like you!” When all the writer can profess to know about you is that you like to read. What a risky assumption.
Join me in saying yuck and finding a better book to dull the experience.