Detective Conan - Vol. 51 Ch. 528 - Russian Blue

[MangaDex]

 

[OsTB]

Kogoro took weeks to suspect Conan was assisting him in cases but suspected the cat immediately after meeting him i can't do this anymore

 

[Aboiaboi]

Well, unlike the cat, conan intends to hide it and intentionally misleads him, so there's that. There's been many times in the beginning where he suspected him, but at this point he just goes along with it.

You may argue that it is really obvious though, but the author designed this story in a way so kids could easily follow the story, so they made he made it more obvious for us readers, but in 'reality' it's a lot more subtle.