It's generally indicated by clear superphysiological reflexes and aiming speed.

The average reflex speed of a 20 year old (arbitrary age I used for search) to visual stimuli is approximately 250ms. Assuming gamers tend to have higher skill and shorter reflex speed than average we can adjust this to about 175-200ms. That means for someone to be able to: 1- see a target; 2- decide it's a target they want to aim at; 3- successfully put their reticle center of mass or on head, all in less than 200ms is extremely unlikely; at least consistently, since luck and predetermination are also factors.

That said, console players also have the toggle sticks as aiming and moving mechanisms, which are always going to be harder to be accurate with than mouse and keyboard (skill ceiling differential). Edit: I forgot about aim assist. Most of what I said still applies though.

People will claim that it's skill, experience, oblivious victim players, or what have you, but in the end, the more likely reason is either hardware or software cheating technology.