That's a bit difficult because I already go into anything from The Onion knowing it's intended to be humorous/satirical.

What I lack in ability to recognize satire or outright deception from posts written online, I make up for by reading comment threads: seeing people accuse things of being fake, seeing people defend it as true, seeing people point out the entire intention of a website is satire, seeing people who had a joke go over their heads get it explained... relying on the collective hivemind to help me out where I am deficient. It's not a perfect solution at all, especially since people can judge wrong—I bet some "omg so fake" threads were actually real, and some astroturf-type things written to influence others without real experience behind it got through as real.