When you pour a comericial beer you leave about a quarter inch of beer in the bottom of the bottle out of habit.
+1, I do it every time. I also immediately rinse out a bottle after I open it, even if I can't re-use it.
Every single room in your house has beer bottles in it - empty or full. (my laundry room looks like a recycling facility).
You get sad when you see an empty carboy laying around.
You proudly show off your fermenting carboys to friends, family, acquaintances, neighbors, the mail man, your dog - anyone that happens to be near your house.
If you are not at your house, you show people pictures of your fermenting carboy.
When you have an entire fridge full of beer ranging in styles, yet you still need to brew this weekend.
You go to the home brew shop for $5 worth of hops, and end up leaving with $40 worth of brewing equipment.
When you hear "mashed potatoes", your first thought is "I wonder what temperature they mashed at?"
When you buy a kegerator setup without having an extra fridge. (Or was that just me?)
When telling someone what kind of beer you have is a 10-minute conversation.
You get confused when people ask if you have ever fried turkey in your turkey fryer.
You offer to pour all of your friends beers for them. (They will screw up the head)
When you call your fully finished basement "the cellar".
And above all, at least for me...
When you are willing to make a 5 gallon batch of beer, and only drink a few bottles because you give the rest away. (But ask for the bottles back, of course).