Is there any possible way to know whether a part is within your line of sight? I have looked around in the Wiki, and I haven't found anything of the sort... So if there is any way to create a method that can detect objects within your sight (detect whether the player can see the object or not), that'd be nice to know! Thanks in advance!
Try using raycasting. If you're in first-person, have rays cast off of the Player's head in the directions of their field of view. If the rays hit an object, like a part, then it means it is within your field of view. Make sure the rays cast in visible areas, and nowhere else. That's my way of solving this, but since I don't know much about the Raycasting API, you'll have to resort to the wiki for more information on how to do this.
This is the best way to do it in my opinion, but there are many other ways, and I encourage you to explore more before coming to a decision.