Originally Posted by John Mendenhall

I'm in favor of a filter for the Board that can distinguish between the contraction for "it is", which is "it's" with an apostrophe, and the posseive of "it", which is "its" without an apostrophe. And will make the correction automatically.

Ask yourself, "Can I substitute "it is" in the sentence?" If you can, then use the contraction "it's". If not, then use the posessive form "its".