For Americans:
When you don't have to do a centigrade conversion to Fahrenheit.
When you are no longer bothered by "roundabouts"
When you talk on your "mobile" and not your cell phone.
When 85degrees F is a cooling trend
When you think the NY Times and CNN are a bit too conservative
When the Gulf News editorials are "spot on".
When you have no clue who is in the World Series, or care or more excited to watch Manchester Utd. play Arsenal than Dallas vs. Washington or Green Bay vs. Chicago.
Not knowing who is in the BCS
When "March Madness" means it will start heating up rather than what it means.