OK, Lillyofthevalley is hogging this topic, but maybe I can keep it rolling.
You know you're an INFJ when:
Every now and then someone says to you, "I can't believe you just said that." (I once had someone tell me "If I had known you were really like that, I would have invited you over much sooner."
You're emotions are usually on lock down in public.
It's rare, but if someone or something pisses you off enough, you can turn into a snarling monster. You are not out of control, just letting the pressure blow, meanwhile they are hiding the knives...lol.
Someone close to you says "See, I know you like the back of my hand", and you want to give them the two fingered eyeball poke because you know they don't have a clue who the real you is.
You show up for work early 99.99% of the time, when no one else does, but the one time you are 5 minutes late your boss thinks he needs to reprimand you. This causes you to fume for 2 hours. You know if you were the boss you would take prior history into consideration before remarking, at the same time you realize that not only can't this guy see outside of his little box, he would run back into his little box if he did. INFJ's have a bigger box, anything ouside of it is reason to contemplate and observe....and so our box expands.
You wonder about people who are 'different' while others cross them off without a thought.
You are more likely to slip a $5 into a homeless woman's pocket when she isn't looking but others make sure the woman knows they're donating.
You don't eat hot dogs for 3 years, then you eat them for 3 months straight. You are willing to try odd foods.
You take pride in your achievements and want to be acknowledged just like everyone else does. However, if they want to put together an event for this reason, you don't want to go.
Compliments on your creativity are nice but no big deal, in fact, too much praise makes you uncomfortable. Deep down, you know what you are getting complimented on is just a drop from the bucket nobody knows about. But once people know you're particularly good at something, they start asking you to volunteer your efforts. You balk at this because you're creativity is yours and only yours, to give only on your own terms, when you want to do it.