(This is written from the perspective of a Brit who has been living in the crazy US of A for the last two years - cuz that's what I am!)
You know you've been away from home too long when...
You wake up after a particularly erotic dream about wine gums.
You don't feel immense guilt pangs when someone else packs your groceries in the supermarket.
You finally realise that when they ask you "paper or plastic" they're not really asking if you want to pay by cash of credit card.
You complain that a gallon of petrol is now over a dollary fifty.
You put on a fake Texas accent just so that strangers you meet don't insist on telling you just how much they want to visit England some day.
You realise there's something odd about the way you've been spelling color recently... Damn.
You learn the secret truth about barbeque.
You are no longer surprised when someone overtakes you on the hard shoulder (Texas Drivers!)
You drive 1/4 mile to buy a bottle of milk.
You begin to understand the deeper meaning to Gilligan's Island.
You wait until you're out of small arm range before you flip the bird at the car that just cut you up.
You call 'giving the finger' 'flipping the bird'.