Search warrants are frequently sealed and not accessible to the public until after they are served, so that someone checking public records can't be tipped off that law enforcement agents are headed their way and hide evidence or flee.
Calling 911 or a local police department is the best that you can do to verify that a search warrant someone verbally states that they have is bona fide (although looking through a peep hole in a door at a document is a good second best solution). Neither are ideal or perfect solutions.
You can trust that if you called 911 or the police that this is who you are reaching. Given the time urgency 911 would probably be a better choice. You can't trust that they are telling you the truth and law enforcement is allowed to lie to achieve lawful criminal justice ends in most circumstances in the U.S. (at least as a matter of constitutional criminal procedure, state law may vary). This said, you don't have other choices, and if you resist you are likely to end up dead from a barrage of police bullets.