The role of the Senate in the confirmation process is defined in the Constitution. Article II, Section 2 provides that the President “shall nominate, and by and with the Advice and Consent of the Senate, shall appoint high government officials.” Positions requiring confirmation are specified by statute. Here's a PDF listing all the executive branch positions that require Senate confirmation.
Sentence
Had Donald Trump's nominated El Chapo Guzman for head of the DEA, he probaby would have been approved by the Senate.