If you can't discern sarcasm through text, then either the internet has eroded your literacy and interpretation skills to the point where you are no longer viable as a member of an intelligent conversation, or you are so dense that you should never offer a response to anyone in the firs place.
Seriously. Sarcasm is sarcasm, spoken or written, and before message boards this was not exactly neurosurgery.
Satire was a celebrated literary art long before the invention of the internet.
Pay more attention. Read more books. Suddenly, you may find the explanation of someone's sarcasm almost completely unnecessary.
However, I suppose that the internet has really contributed to the evolving and eventual superiority of language, right?
(That was sarcasm)