For me Kuro's face on the cover already gave it away but the chapter 1 illustration + the fact Shiro was pursuing Kuro + Shiro's "I'm all grown up now" talk in ch3 which is like, THE standard trope with younger tops + their dynamic, with Shiro's overall mischievous, kind of boy-ish personality as well as Kuro's tendency to give in to Shiro's wants just made it obvious Shiro would want to tap that and Kuro would just say "sure"

It was so obvious the doggy is the bottom, idk what y’all on about ╮( ̄▽ ̄)╭