You know you're obsessed when...
... The only way you can focus on the conversation you're having and/or stay on topic and not break into ranting and raving about Tsubasa is to chant 'Don't think about Fai, don't think about Fai" under your breath many times over.
... You get in an extended discussion about what kind of underwear some of the characters would wear. (I actually did this once, but not with Tsubasa. Though it's only a matter of time...)
... You really think about how tall Kurogane is. (Seriously! He's a frickin' giant! ... at least compared to me... And Syaoran, I guess.)
... Characters from series other than Tsubasa start to remind you a lot of... Well, characters from Tsubasa.
... You notice that some characters from some series look quite like some characters in Tsubasa, and spend the rest of the episode/chapter of that series thinking about said similarity. (Faust... Fai... Faust... Fai... It works in my head...)
... When reading, you start imagining the characters in the book as characters from Tsubasa (Lyra and Will are now Sakura and Syaoran.)
... You seriously consider cosplaying as Kurogane at next year's Anime Convention so you can 'match' your cousin/friend going as Fai...
... You spend hours designing new outfits/forms for Fai and Kurogane Syaoran and co. (This especially applies when you are doing this instead of something important.)
... You invent new worlds, each with their own laws, politics, animals, races, etc. just to have an excuse to draw more of/write about Fai and Kurogane Syaoran and co. in their new forms/outfits.
... You ask yourself, 'What would <insert Tsubasa character of choice> do?'
    ... And make yourself an appropriate bracelet/jewerly afterwords.
And, of course, the obligitory 'You post in this thread' point.
And I think I'll stop here, as I've lost the ability to type coherently, and I think I stopped being slightly humourous/sensecial awhile ago.
... Thank-you, thank you, I'm here 'till Thursday, which makes saying that funny 'cause where I am it's Thursday right now...