When trying to identify wall hackers, you have to remember that every player has a few key tools at their disposal that if used properly can make you look like a hacker.

Minimap/Radar: If a teammate sees them, even if your computer sees them through smoke or a flash, they will show up on the minimap. Also if someone doesn't move from a spot after killing someone else, it's very easy to quickcheck their last position or prefire into it as a precaution. MANY pubbers will camp in a spot/area after they've killed someone.

Latency: Often times you may think you've safely made it behind a wall, but the other guy saw you trying to ditch away.

Shadows: I play with them. Some people might not due to performance. These can play HUGE roles in spotting someone hiding, or even seening someone through a door/wall as the shadow may glitch and reveal them.

Gun barrels/body parts: I've seen someone so many times looking around and their gunsticks through a wall or a foot glitches in a door.

Sound: The biggest one. If you've never owned a pair of gaming headphones, you can't understand how much sound plays in tracking people. The sound of them jumping, switching weapon, reloading, everything comes into play. It's so easy to track people with a good pair of headphones.

This is why when banning for wall hacking, you have to be especially careful, because all these other factors come into play.

This being said, I've spent many many rounds tracking Piller. I haven't seen anything yet that really makes be believe he's wall hacking.