The judicial branch of the U.S. government declares laws unconstitutional. The federal courts of the judicial branch have the sole power to determine the constitutionality of the law, interpret the law and apply the law to cases that are brought before it. Article III of the U.S. Constitution established the judicial branch to balance the powers of the legislative and executive branches of government.
Laws are not automatically reviewed by federal courts; the courts only try certain cases. A party must bring a suit in federal court and give evidence that the party has been harmed by the application of a law.
Most federal cases are brought first to the U.S. district courts, and these decisions may be appealed to one of the 13 U.S. courts of appeals. From there, a small number of cases are actually heard by the Supreme Court.