- Check the source. This helps in two ways. First of all, if it's a site that only posts fake news -- such as The Onion, The Daily Currant or Reductress -- you'll know it's fake right away. And yes, I have sadly seen many people post things they thought were real from The Onion. You also want to check the source to ask yourself why this site is the only one you're seeing this story on. For instance, do you really think a website called "Story Carnivores" is going to have an exclusive scoop about an eighth Harry Potter book? (Hint: The answer should be no.)
- Think it through. Is what you're reading even remotely plausible? Take the article that's been going around about Samsung paying Apple a billion dollars in nickels. As Snopes points out, this is entirely illogical for a few reasons: The lawsuit isn't finalized yet, so there's no way they would pay it off so quickly, and 20 billion nickels would be A) basically impossible to acquire and B) definitely impossible to fit into 30 trucks.
And that's pretty much it! If it's from a reputable source (or better yet, you're seeing it reported by multiple reputable sources) and it seems plausible, it's probably true. If either of these things are not the case, it's probably not.