Answer:
Yes, the federal courts can declare legislation passed by Congress and signed by the President unconstitutional, but someone who has standing to challenge the law must file a suit first (the Supreme Court can't just pluck a bill off the President's desk and start critiquing it). In order to have standing, an individual must be personally, directly and negatively affected by the law in a significant way and there must be an issue that can be remediated by a court. Not all legislation lends itself well to judicial review.