you need raycast!
A RayCast represents a line from its origin to its destination position, cast_to. It is used to query the 3D space in order to find the closest object along the path of the ray.
RayCast can be configured to report collisions with Areas (collidewithareas) and/or PhysicsBodys (collidewithbodies).
Create a StaticBody under that create a CollisionShape. Set the CollisionShapes Shape. Best put your geometry in the StaticBody or CollisionShape container your just created.
Create a RayCast and put it under your camera. Do you see the faint white line? This is the ray that checks for collisions. Write a script ... check RayCast for iscolliding( ) ... get the collider with getcollider( )