Look for statements of emotion or opinion that seem out of place in the given context. If a writer's supporting an opinion that seems completely illogical, there's a good chance he or she's being sarcastic and attempting to convey the opposite position.
Keep an eye out for hyperbole, or exaggerated phrasing. Writers often overstate an opinion to reveal its logical or ethical flaws.
Watch for italicized, underlined or bold-faced words. These show that the writer is trying very hard to emphasize whatever point he or she's making, which is in many cases a sarcastic one.
Think about the personality and typical writing style of the writer in question. If he or she normally writes dry, boring prose, a statement you think is sarcastic might actually be sincere. On the other hand, when reading crafty writers like Mark Twain or Oscar Wilde who are known for their wit, keep a constant vigil for outlandish or peculair statements.
Consider whether the text you're reading should contain sarcasm or not. For example, textbooks and legal records won't contain sarcasm, but op-ed columns and position pieces likely will.