I really need help. I don't understand how to check what you're looking at.

It is very important for my mod to make it so that when the player looks at a certain block/entity from my mod, a certain message is displayed on his screen.


I know how to display messages and have already written the code for this, but I don't understand how to activate it, because I need the program to start NOT when the player clicks on the block, but when he looks at it, without clicking the mouse. I have not found such a trigger, and all the alternatives that I came up with myself are very heavy on the PC and Minecraft crashes.