You try to build your own TARDIS. [My brother's friend did this, it was really awesome but you couldn't fit more than one person inside it.
Whenever you get stomach cramps, you clutch your abdomen and yell "I'm regenerating!"
When someone tells you the date, you reply "Again?"
You're terrified of people with Bluetooth earpieces.
You spend hours trying to work out "reverse the polarity of the neutron flow."
You are convinced that you are a Time Lord/companion who has lost their memory/the only surviving Human-Time Lord Biological Metacrisis.
When it snows, you build a Snow Dalek instead of Snowman.
You watched Nan's Christmas Carol just because it had three whole Doctor Who actors in it [Catherine Tate, David Tennant and Roger Lloyd Pack]. (Though this might just be for people who live in the UK, as I don't know if you can watch The Catherine Tate Show in the US.)
When anyone knows anything about Doctor Who, you automatically say something along the lines of "You are my new best friend" thinking about the episode Planet of the Dead where the doctor says the exact same thing to one of the scientists :)
You look for any keys that may belong to a Tardis.
You avoid lighting where it casts more than one shadow.
You want or have a blow-up Tardis. (I have yet to get one.)
You'e dressed up as ten.
The converse you have is the kind of converse ten wore.
You look for small blue whirring flashlights.
You made your own "psychic paper."
You know every actor's name that has ever played the Doctor.
You don't blink in churches, graveyards, etc.
When I was in D.C., I saw a guy that dressed as the Doctor and was twirling a blow-up Tardis around. Since I was on an escalator going up with people behind me, I could not take a picture, nor hug that guy to death. It's my biggest regret...
To the point where I'm shopping for clothes and I pick things out from the racks and look at them and honest to Rassilon sit and think to myself things like "Oh, this would look good on Sarah Jane" or "Nyssa would like this".
Oh, and the fact that I keep throwing things like "honest to Rassilon" and "oh my Gallifrey" into everyday speech. That's a big one.
Hm... the first comment I had on my mum's new haircut? "Looks good. A little big like Tegan, but not in a bad way. She may have been obnoxious, but she had great hair."
I've also got loads of weird phobias thanks to Who. Water, statues, gas masks, shadows, nanotech, new popular products (a la Bubbleshock of the Sarah Jane Adventures), laser tag (also SJA related), cracks in walls - you name it, I've got it.
And I often think to myself in any given situation what a particular Doctor (usually Five or Two, but sometimes Nine or even Ten or Four) or companion would do.
Also it's gotten to the point where I feel as if I literally have a third hemisphere of my brain to run Doctor Who thoughts at all times, no matter what else I'm doing. It's the ultimate in multitasking, only mostly useless, except for mystifying my non-Whovian friends.
Oh, and also, I can list every single on-screen companion in order and tell a bit about them, and most of the actors/actresses who played them, excluding people who were only there for one offs or two episodes. Well, all the on screen companions, and Lucie and Charley.
And there's also the fact that a good ten or more percent of what I say is an obscure quote, which often leads to me rattling off a section of dialogue from like, Tomb of the Cybermen or something like that, which confuses the hell out of everyone.
When you see those little bits of dust in a beam of light, you think 'vashta nerada'
Bow ties are cool, and now so is a fez
You don't go out on Saturday nights
You listen out for the TARDIS materialising in your garden
You always write TARDIS in capitals
You know TARDIS means Time And Relative Dimension In Space
You know you will be a companion one day.
You feel the need to go inside a police box when you see one, just to check it isn't the TARDIS