If the Supreme Court decided that Marbury was entitled to his commission how could it be sure that the Executive branch would deliver it?

They couldn't; that created a significant dilemma for Chief Justice John Marshall.

The Supreme Court has no enforcement power over its decisions, so there was no way they could compel Madison to deliver Marbury's commission, which may be one reason Marshall denied the Court had original jurisdiction over the matter. If the Court issued the requested writ of mandamus, and Madison refused to comply (a theory tested when Madison refused to answer the Court's inquiry as to why it should not issue a writ for the commission), it would weaken the power of the Judicial branch.

When a Court decision involves a question of documented law, the courts generally make a ruling, then remand the case back to the Circuit Court (US Court of Appeals) or state supreme court to ensure the decision is carried out. Otherwise, only respect for the Court and established protocol encourages compliance.

Case Citation:

Marbury v. Madison, 5 US 137 (1803)