The same way Osiris could tell someone (his own son in this case) was with Seth is the first arc.
Left over semen in his inside since he doesn’t need to evacuate like a human does (a piece of information that was given by the Beard god when he raped Seth a few chapters ago).
And there is probably abrasion and cut from when he was forced by the Caravan people. It may have healed, but it still let someone marking.
And at last, and more importantly there is the running trope of all those BL that think, just like for a vagina, that the moment someone has been penetrated once, it magically become loose to the point that one can tell just by touching. Which is physically not how things work. But it is a stubborn (and unhealthy) trope.
Osiris just knew, I think? Because he impaled Seth right as he walked into the room. Then you could see the semen from Horus on the spearhead that was coming out behind him. He wasn't inspecting his backside or anything at the time.
Yes that is what meant.
When Seth was still a good, his body was sad and Osiris put wine seed in that body. Which he used to feel what was inside. He implode Seth because he knew what was in it and was mad about it and wanted it out. (He has punished Anubis for kissing Seth so you bet he can hurt Seth for « letting another man have him »
In any case he did need to inspect him. He was « feeling it from his plants.
When Furrball mentions god don’t defecate, it is Emily that things that goes in, stays in. Even for a demigod. So when Horus touch Seth he might have felt it. The fact he did it differently from his father doesn’t change the fact that both could « feel » thing inside and therefore came to a conclusion.
My comment wasn’t about if the way they proceeded but what they discovered.
(⊙…⊙ ) wtf also how tf does he know that seth had someone else inside of him?!