Do you call each other names that would make a stranger think that you hate each other? Do you do things for each other? Do you share secrets? Do you share stories?
Acquaintances will know the surface you friends will know the real you.
That's the kind of question where almost everyone will give you the same answer.
A friend will say you're friends, because you are.
Non-friends will say you're friends; because either they want to take advantage, or don't want to offend.