You're confusing wallhacking with noclipping. Wallhacking is being able to see through walls, like drawing an outline around all characters that renders with highest z-order, or making wall textures transparent.

It does not result in any server-side-detectable difference in behavior other than the hacker seemingly being more aware of their surroundings than they should, which can be hard to determine for sure. Depending on how the hack is done, it may not be detectable by the client either, eg by intercepting the GPU driver calls to render the outlines or switch the wall textures.