When you make lots of random noises in private, depending on your mood, and sometimes make them in inappropriate public settings.
When you can break into song at any given moment, including in the middle of a conversation.
When your wife knows to stop and check every few moments to see if you are still listening to your conversation.
When you use a bluetooth earpiece with your phone and you zone out the conversation and go on with your life while the person on the other end wonders what in the world you are doing.
When people regularly ask "Where did that come from?" in a conversation because you took so many detours with our thought before anything came from you mouth. Something like "Do you want to go to the avengers movie?" And you think avengers, doesn't that have Captain America? His last movie sucked. It was a failure. Oh crud, speaking of failure I forgot to call my client back about his tax return. Oh man, it was due today. I wonder if I can print an envelope from stamps.com with today's date and mail it tomorrow and avoid a penalty? I may need to buy him a gift certificate. Maybe Olive Garden would be good. I love their Tuscan potato soup. Maybe we can make some potato soup tonight. And you reply "Do we have any potatoes?"