@veiny ah dih betas can't smell pheromones. The tope knew the bottom was in heat because, he was groaning? In the panel he say "ah!", and that sound alerts top.  He cannot smell pheromones so when he looks back he sees the bottom is having a difficult time maintaining his composure, so he knows that bottom is having rut.  Hope it helps!

@The_otaku_fujoshi how can he smell his pheromones then?(is he even smelling them? Or is h...