Qalb can occur in prose as well as poetry. A brief prose expression may be reversible; in poetry, the device may span two hemistichs, occur independently in each, or apply to a selected segment.
“كُلٌّ فِي فَلَكٍ” demonstrates the mechanism in a compact phrase; longer verse brings rhythm, grammatical sense and poetic structure into the analysis.
A reliable test has three stages: identify the claimed span, reverse its letter sequence, and check whether the original sequence is reproduced exactly under the relevant conventions.