What evidence supports the idea that Keats is talking to a Grecian Urn in his poem?

Keats neatly sandwiches his stanzas between the first and last lines which both address the urn:  Thou still unravish'd bride of quietness,  and  O Attic shape! Fair attitude! with brede...Thou, silent form, dost tease us out of thought As doth eternity

The first stanza is an introduction, addressing the urn itself.  Who are these people and what are they doing on your sides? What can it mean for those of us just now laying eyes upong you?

The second stanza introduces the theme of imagination, but by addressing the musician on the urn.  The music we can all hear may not be liked by everyone, but the music we imagine will please us all simply because we imagine that which we like. 

The third stanza addresses the lover, the musician, the trees which will forever remain in love, young, playing without tiring, and full of green leaves for eternity.  There is nothing to worry about here in this stanza...no sunburn, parched tongue, or broken hearts.

The fourth stanza questions the priest leading the cow to sacrifice and the absent townspeople about where they are going and what they will do, which all leads to the question: What is the truth about all of this?  We can see and appreciate the beauty, as will many to come.  "Beauty is Truth, Truth Beauty,"—that is all Ye know on earth, and all ye need to know.