
Originally Posted by
emsr2d2
I agree with Bhaisahab that "Have you finished?" is the correct term and that can be said both formally and informally.
I think with either of the phrases, it depends on how you say it. If you approach an older person, or someone who you don't know very well, and who you would generally speak very politely to, it's possible to make the question sound polite simply with your tone of voice or possibly the addition of "Sir" on the end if you are a waiter.
"Are/Have you finished, Sir?"
"Yes, thank you."
"OK, I'll take your plate. Did you enjoy your meal?"
"Yes, it was delicious, thank you."