In many cases the any court (in almost any country influenced by the French law) interprets the laws and confirms if the 'new laws' is consistent with the 'old laws/ more basic laws' and not in violation with the basic premise of the constitutions. Of course this is a over-simplification but this is at the most basic the function of the courts.
But I believe the US uses Common Law (English) and not Civil law (France and many other places) system for these matters. So judge can in the US (btw IANAL) decide if a law is valid or not based on constitutionality and previous laws.
This is absolutely correct and is an important distinction and I do confess that my statements were not very precise from that regards. However, I had a long discussion with a law professor at Berkeley where she did agree that the US Common Law is highly influenced by the French Civil Law, though many law academic in US will take an exception at the statement. Also, IANAL either but am taking a few law courses here.