When you're correcting anyone and everyone that says "Legos" regardless of their age.
When you're editing every question and answer I post to correct Lego to LEGO. :P
When your Lego collection is worth more than your car and/or house.
When you have an entire room in your house dedicated to Lego.
When you want to show every child and parent at the PAB wall how to get the most in their cup, but usually manage to keep your mouth shut.
When you spend more time sorting pieces than building with them.
When the "book" collection is your house is 90% Lego manuals.
When your Lego collection is larger than your kids collection.
When you buy multiples of sets "just for the parts".
When it's mandatory to walk down the Lego isle in any store that has one.
When you can't complain about your wife's shoe collection because it pales in comparison to your Lego collection.
When you have developed a legitimate hatred of stickers in Lego sets. Either because it seems "cheap" or, no matter how perfectly you apply them, some will wither and die with age.
When you've rolled up to the check-out with a shopping cart overflowing with storage drawers and/or containers.
When your fingertips are numb from pulling 1000's of Lego pieces apart, but it doesn't stop you.
Ok, I should stop now...