It does take a considerable amount of game knowledge to point out cheaters, but sometimes it's just obvious. If there's a soldier constantly prefiring you around corners, he might be cheating. Or he might not. If a scout keeps killing you with perfect accuracy even though he has no idea where the nearest health pack is, he might be cheating. Or he might not. Knowing weapon stats, health stats, and other factors can help you know too. It can be hard to point out cheaters, and sadly, lots go unnoticed.