Answer to Question 1
There is a variation in composition and density, and thickness. Drilling into the sea floor and continental crust, and detection of ophiolites, where oceanic crust is accreted to continental material, provide opportunities to study the crust directly. Seismic reflection and refraction allow them to image the lithosphere.
Answer to Question 2
Drill-hole data, surface of outcrops of rocks that formed at depth but are now exposed on Earth's surface and rocks brought up from depth within volcanic rocks.