There is a function Tailor()->is_canvas() to see if we are on the Tailor canvas (currently editing the page).
There is another function called Tailor()->is_tailoring() but this is not to check if current post was edited with Tailor. I don't understand the purpose of this public function...

It will be nice if we could check in the WP loop if the post was tailored, with a public function like Tailor->is_tailored(). Actually I do that with:

if ( false == get_post_meta( $post->ID, '_tailor_layout', true ) ) {
   // $post not tailored
} else {
    // Tailored $post !
}