Use the node “Single Line trace by Chanel”. This node trace a line between a start point (the camera) to another point (the object or a distance for exemple) then retrun what the line hit at the end. I just take this value and ask if the actor who is hit by the line is equal to the actor i want … And that it, if its true you see the actor, if not you dont. Youcan ignor certain actor (transluent material for exemple) and the line pass trough.
After that I use some math i saw in the content exemple for checking if the actor is in focus in the midle of the screen :
To finish I just ask if this 2 booleans are true (the AND Node). If both are true, that mean the object is not obsured by a wall for exemple, and if it’s in the focus of the camera :).
I hope this helped, sorry for my bad English 