In the case of Hodel v. Virginia Surface Mining Association (452 U.S. 264 (1981)), the Supreme Court established a four part test to decide whether or not a Statute violates a State's Constitutional immunity (via the 10^th Amendment) from regulation. To be legal, the Statute:

• Must regulate the States as States. (as opposed to people in those States).
• Must address matters that are indisputable attributes of State sovereignty.
• Must directly impair integral operations in areas of traditional government functions.
• Must not be such that the nature of the Federal interest does not justify State submission. (This part is debatable. It can be argued that even if there is a national interest, you still can't violate the 10^th Amendment).