How do they know you haven't trained an AI to get headshots? The cheats often break the bounds of what is realistic in games, whether it is allowing you to see through walls (server shouldn't be sending enemy positions that aren't in view), going too fast (server should speed check pplayer positions), getting items they shouldn't have (server should do inventory sanity checks), etc. Other than that, look for signs of automated movement/things unrealistically precise for a human to do. Eventually the cheating will just be moved to a separate air gapped computer running AI on the video feed. Client side is an invasive, broken, and malicious concept.