Players are permitted to hit the stick of the player possessing the ball. Stick checking is an excellent strategy for forcing a turnover, but it must be done properly for the best results and within the rules since this is a heavily scrutinized maneuver by officials. There are several circumstances that determine a legal stick check.

Any check that occurs within playing distance from an opposing player that is performed in a reckless, dangerous, or intimidating manner must be called.

That statement from the 2019 USLacrosse rulebook provides a good idea of what the refs consider judge a stick check. The ball/stick must be below the shoulder of the player possessing the ball to execute a legal check. The check may only be made between the heads of the sticks. The stick of the player making the check must not pass within the sphere (the imaginary spherical zone around a player’s head), nor towards or across the body or head of the opponent. The opponent must have the ball in their stick unless both players are pursuing the ball under an equal opportunity and the ref considers the stick contact nonincidental. Yes, like all sports, a lot is left up to the ref’s interpretation of the rules and the play.

Here’s a good video outlining an effective stick check.