Answer:
The Judicial Branch, which is lead by the US Supreme Court, checks the power of the Legislative Branch through the use of what is called Judicial Review. This allows the Judicial Branch to decide if a law that Congress passed is constitutional or unconstitutional. Further, it allows the Judicial Branch to "define" that law by answering questions about it that are not spoken to directly in the regulation itself.
This power is not unlimited, however. No court can consider the constitutionality of a law unless the law is a relevant part of a "case or controversy" before the court.
Chief Justice John Marshall clearly affirmed the Court's right of judicial review in the case Marbury v. Madison, (1803), when the Court declared Section 13 of the Judicial Act of 1789 unconstitutional.