Hi DB07, welcome to the forum.
Best indicator if the body and neck are sanded enough are to run your finger tips over the entire surface, you can generally feel machine marks that haven't been sanded properly.
After you pass that test rub a damp (water) clean rag over the whole body/neck and then machine marks or scratches should be more obvious to the eye.
Start on 180 grit and move though to about 320 or 360 grit