Every ceiling in your home is colored with unfortunate krausen incidents
You put more time and effort into maintaining your home brew equipment than you do your car. 
You saw this post.
You know how to pronounce diacetyl.
You know more beer styles than you have ever sipped on.
You can argue the between the difference between an Ale and a Lager.
You call a Turkey Fryer a Boil Kettle.
You walk by a cooler and think, “How efficient can I make this into a Mash Tun?”
You buy beer based on how nice the bottle is.
You drink a beer by checking for haze, smelling the beer prior to sipping it, staring at foam like it is a wonder from god, and asking what hops are used? Asking yourself what base malt was used? Asking yourself can you clone this? Then getting your cell, tablet, laptop, or desktop to look it up to see if you were right on any of the previous.
You have brewing software on more than 1 device.
You book a vacation then you look up what regional breweries are near by.
You own more than 1 book on beer.
If, as a Belgian, you know exactly what 170 F is in centigrades 
You’ve seen “you might be a homebrewer if” at least a thousand times…
Or at least twice in one week.
you can perform grain to gravity calculations in seconds without a calculator.
you have a screen on your phone strictly reserved for homebrew related apps.
you post in “you might be a homebrewer if” threads despite haters hating in that same thread.
When you walk around Home Depot, most of the things you see may be handy in the brewery.
…you hear mention of “river otter” and get excited about a new English malt…
Brewed an American Light Lager and actually took it to a club meeting to show it off…
You keep scent-free hand soap in at least the kitchen and powder room, if not the upstairs baths and the laundry room as well, so that the soap scent doesn’t get in the way of being able to smell your beers.
It’s not?
You walk into any store and ask which of these products could I use in my brewing?