I'm currently working on some Rendering Code, and I wondered if Graphics2D.draw(Shape) is performed the exact same way onscreen as offscreen. If it is, is there a way to check if my shape (a Line2D) is onscreen?
Depends on what you mean by "exactly the same way".
As far as calls to draw-something() from your code are concerned, they take place normally. What is different, somewhere in the rendering chain the condition that the shape does not actually fall into the actual rendering area is detected and no pixels are actually output for the shape.
As for detecting if something is "on screen", that depends what you define the screen as. Reasonably you would define it as the area described by your Graphics object (that may well be only part of the screen, or completely off screen, heck it could be a printer!).
Provided you have not changed the clip area of a Graphics instance, getClipBounds() gives you the rectangle where graphics are actually rendered. If your graphics comes from a Swing component, you can also substitute the components dimensions for the rendering area.
Once you have the actual Rectangle, you can make use of the methods provided by Shape to detect collisions (contains(), intersects() etc.).