How is the Supreme Court able to check the power of Congress?

The Supreme Court uses judicial review to declare actions by the President or Congress to be invalid if they are contrary to the Constitution. The Constitution is the supreme law of the land and no presidential act or congressional laws may conflict with it. The Courts are the interpreters of the laws and as such they interpret the Constitution and laws to decide if they conflict with one another.

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.

Chief Justice John Marshall clearly affirmed the power of judicial review in the case Marbury v. Madison, (1803), when the Court declared Section 13 of the Judicial Act of 1789 unconstitutional. It should be noted that the US Supreme Court, for the most part, determines what laws to review based on decisions made in lower Federal Courts. In most cases the Court waits for a case to be presented to them. It then can reject hearing the case or depend on the ruling of the lower courts.