Trigger something when a Part is being seen by the Player's Camera POV / Head part ?

Though i don't know if it involves magnitude, raycast or whatever- I just cannot make any sense of how they are able to do that, even in free models- neither i can't make a use of "Camera:WorldToViewportPoint" and don't even know if its meant for that. I can't imagine it being simple but im wondering if anyone even knows how to do so.

It's been a few days for me trying to figure it out and this is all im asking for so lo