The law plays a defining role in federalism. In practice, these are not clear lines. The Supreme Court has had to determine when state activities trespass on areas that have explicitly been preempted by the federal government and when federal acts encroach on states' reserved powers. Over the course of U.S. history, judicial decisions have punctuated the changing relations between states and the national government.
Tenth Amendment gives an annotated discussion of the amendment with links to relevant cases.
Most of the rulings that distinguish between federal and state domains have hinged on interpretation of the commerce clause. Decisions on cases that fall in the overlap between state police power and federal authority under the commerce clause continue to evolve. Indeed, after decades in which federal authority over virtually everything prevailed on commerce clause grounds, one of the most recent Supreme Court decisions in this area overturned a federal law related to guns near school property in part on the grounds that it strayed into state police powers. The fourteenth amendment has also been an area where courts have actively defined the federal-state relationship. The courts serve as the referees between governments, interpreting and expanding on the relationship originally delineated by the Constitution.