There is a time lag between the impact and its apparent effect on the tackled player.
The tackled player moves farther or in a different direction than expected from the momentum of the tackle.
The tackled player nurses a body part other than where the impact occurred.
The tackled player falls in a posture resembling a drawn bow, counter to normal reflex mechanisms to protect the body in a fall.
The challenged player falls to the ground before any impact occurs.
Based on a 2009 study by Paul H. Morris & David Lewis from University of Portsmouth, UK: Tackling Diving: The Perception of Deceptive Intentions in Association Football (Soccer)
There are different circumstances when a player must be cautioned for unsporting behaviour including if a player:
1. attempts to deceive the referee e.g. by feigning injury or pretending to have been fouled (simulation)
An action which creates a wrong/false impression that something has occurred when it has not (see also deceive); committed by a player to gain an unfair advantage.
Act to mislead/trick the referee into giving an incorrect decision/disciplinarysanction which benefits the deceiver and/or their team.
Extract from Laws of the Game by the International Football Association Board (IFAB)