My tipoff is when the narrator drops into language that is clearly inappropriate to the characters or setting.

For example, if 15 year-old Catholic schoolgirl Suzy has just awakened the hormones of chaste and pious Father Murphy, a dedicated man who has never had an ungodly thought in his 57 years, the good priest is going to be rather trepadatious. He will be examining his feelings, excited by his new consciousness of his body, but still possibly doubtful and anxious. The narrator can communicate this using fumbling or uncertain language. Father Murphy is NOT going to "unsheathe his throbbing cock and guide it towards her virgin asshole." And Little Suzy, even if she is thoroughly overwhelmed by her lust for the man she loves -- she realizes now, she has always loved, ever since he saved her from her abusive parents when she was nine and brought her to the academy to study under his guidance -- is not going to shout "Do me!" If either of them has heard the term at all, they are definitely not going to spell it "cumming." Neither should the text.

Honestly, I think the line between pornography, or even erotica, and real literature is not nearly as firm as we imagine. But even porn that exists solely to help inexperienced 13 year-olds beat off is more enjoyable, and less dehumanizing, when it takes its characters seriously and presents them as coherent people rather than graceless puppets. Please, pr0n authors of the world, give them their own voice... even if they aren't going to have their mouths free very much.