Thank you!
Right now I'm using an Area2D RectangleShape2D to mark the area of the wall, and then casting an intersection ray from the guard to the player. If the ray hits the wall before the player, it lets the guard know that they can't see the player yet, but if it hits the player then the guard knows they can see the player.
When the whole cone color changes from blue to red, that's my visual indication for debugging that the guard sees the player.
The whole business of redrawing the cone so it's shape reflects what can and can't be seen by the guard is what has me stumped. But I think I'm just going to set that idea aside and work on getting the other game-play mechanics in place.