...when you start making references from anime when you see something in another anime, in real life, etc. Who the hell do you think I am?!
And when you start using weird sounds at the beginning or end of your sentences in real life or online, nyo. There. Di Gi Charat reference get.
Oh, and it's been said, but using random Japanese. That's why I hope I can study it in college so I won't be massacring the language as well as not be considered a Weeaboo.
And finally, sometimes typing things in Japanese characters, with them being real Japanese words, obviously. あかです。 Giving your username Japanese characters is also categorized under this. Hey, if "Kagebouzu" romanized wasn't taken by one of my accounts before it got a name change...
Edit: Oh, and when you spend one day getting everything you need for a cosplay outfit when you're going to the convention the next day, and even though you know the outfit's gonna suck, you still go through with it anyways. Guilty of this, though I went as Marisa Kirisame, and even though there ARE Touhou anime spinoff things, as well as Touhou mangas, she's more or less a video game character, so yeah.