The most widespread telegraph in all unarmed combat is to look directly at an intended target on the opponent's body.
[1] In martial arts that utilise legs as well as arms for striking, telegraphing often involves hip movements used to shift bodyweight.
Ice hockey players tend to telegraph larger, more forceful actions such as slap shots or body checks.
In baseball, pitchers can telegraph their selected pitch by allowing the batter to see their finger position.
This is because those players who do betray their intentions, do so when they fail to suppress physical reflexes (such as blinking nervously when dealt a bad hand).