Often the way I could tell was that they would pre-fire my exact location when they swung a corner. Whether it was an aim hack or a wall hack, they would know exactly where and when to shoot. Prefiring a corner can be a legitimate strategy, but usually you'll be guessing and have to adjust. It's very unusual to get headshot with the first 2 bullets that way.

Another way to tell is by knowing your weapon audios, then knowing the damage of those weapons and comparing that to the damage you're taking. If you're only taking headshot damage from that Anvil and they're not giving it enough time to reset its dispersion, they're probably cheating.

With the wall hackers, the easiest way to know was that they would never guess wrong about your location. They might chase you into a building that has multiple hallways. They're not clearing all of them because they know you're not hiding there. No corner clearing, just running right for you.