You know you're a knitter when...

[Riss]

You've worn a little groove into your left thumb nail from helping your yarn off your left needle...

[DreamWeaver]

You know you're a knitter when...

you'd rather go yarn shopping than clothes shopping
You stay up late at night because you have to do "just one more row"
You have said the words "just one more row"

[scout52]

You know you are a knitter when

you go to the clothing store and see a sweater and think, hmm where can i find that pattern to knit that? instead of just buying the sweater.

[ArtLady1981]

...when you have to clear knitting bags and pattern books off the "spare" sofa and out of the aisles around everything else to seat your "drop-by to see Mom" visitors!

[Zvaella]

You know you are a knitter when

you go to the clothing store and see a sweater and think, hmm where can i find that pattern to knit that? instead of just buying the sweater.

I was going tos ay something similar;

... when you see someone on the street and you think "I have to knit her ___"
... when you find nitting needles under you on your couch, your bed, etc.
... when your stash grows faster than you knit!

[jdee]

I agree with the one about enjoying yarn shopping more that clothes shopping.

[AnotherBecca]

when you have meetings at work and ask your colleague to let you copy her notes later so you can get some knitting done!
(guilty!)

[Rorshach]

and you see a skein of mmmmmmalabrigo and drool.

[msmelody]

When you ask someone you have never meet to go feel yarn, so you can have some idea of what it feels like.

Getting mad because eating, sleeping, and other things use the time you could be knitting.

Looking on the internet at 2am looking for a pattern that you just have to make. (this one looks and acts like me)

When your child's first word is knit. (DH not very happy about it either).

[Silver]

When you have a argument with your husband about why $5000 speakers would be better used if sold off and spent on yarn. (I didn't win, he's keeping the speakers)

Or

When you get the words "Knit" & "Purl" permanently inked into your skin.