Mandelstam : "an arch appears in my muttering".
For me, this is one of the most concise & precise characterizations of the poetic process.
Artists testify to the creative experience as a moment which begins when the art-work coalesces, forms an integral whole. Sometimes the poet intuits this process in advance. It's an act of imagination and synthesis. This is the point : the art work is some kind of organic-conceptual whole.
The poem fits together : its sound, its imagery, its plot, its meaning(s). A new invention, a little world unto itself.
Paintings fit together, novels fit together, musical compositions fit together... & poems - rhythmic incantations - also fit together.
This fitness & integrity of the unique art-work - within its very particular and specific mode - prose narrative, drama, music, visual art, poetry - this wholeness - is its ruling characteristic, without which the work does not really exist.
Of course there will be crossings and shadings between verse and prose, narrative and lyric, lyric and dramatic. Novels & plays will be written in verse, prose will intrude into poems, etc. But this does not negate the primary requirement of aesthetic wholeness, whatever the specific genre or rhetorical mode.
If you start with wholeness, and then look at the particulars of diction and style, you might be able to distinguish more precisely the difference between the achieved poem and the merely textual discourse, scribble, speech, imitation, or harangue.