…You have rearranged plans that would keep you away from the computer to accomodate OT Day. 
…You not only know who all the people on the BB are, but can also correctly identify who Hammie, Barbie, Kendall, Fred, Rhodey, Elliot, George, Cypress, Jade, Tilly, RRB, etc. belong to. Further bonus points if you also know posters’ dogs’ names! 
…You plot ways to veil OT topics in horse-relatedness. 
…You can pinpoint in what forum and on what date (within a month or two) a particular thread appeared, for more effective searching in the archives.
…You can explain how to post a photo to someone else.
…You take photographs of things you never would have thought photo-worthy in pre-BB days, just so you can post them to illustrate a story.
…You know that draw reins are “bad” and auto releases are “good.”
…You gave up celebrating each 1000-post marker about 2000 posts ago.