Q. What is it supposed to be on the show? A. The Miami Metro Police Station. Q. Where is it supposed to be on the show? A. in Miami, Florida. Q. When did we see it on the show? A. In Episode 1, of Season 1. But this is sure to be a regular location, given that it's where both Dexter and Harry work. We are certain to see more shots of the building as the series progresses .   Q. What is it actually in real life? A. An office building. Q. Where can I find it in real life? A. A long way from Miami .    This building is known as the City National Bank building, and you'll find it at 11 Golden Shore, in the city of Long Beach, California.

Long Beach is the second largest city in Los Angeles County, located about 20 miles south of L.A .

Because of its resemblance to Miami, the original Dexter series filmed a huge number of its scenes in Long Beach.

Rita's house in the first season was in Long Beach, as was the home that Dexter & Rita moved into after they were married. And if you could see the ocean in a scene from Dexter, it was almost certainly shot in Long Beach

In fact, this building is just a few blocks west of the Shoreline Village area of Long Beach, where Dexter filmed a number of scenes over the years.
 
Located at the base of the 710 freeway, the building is somewhat hidden from view by circular freeway offramps and trees.
But bear in mind that only the exterior shots of this building were used for Miami Metro.

The interior shots of the police station are filmed on a set built at the new studio.

Q. How the heck did you figure out where it was?

A.  I am very familiar with the city of Long Beach, and I recognized this building the minute I saw it.
 
I knew it was located just south of Ocean Blvd, but I needed to check its exact street address, which turned out to be 11 Golden Shore.

Here is a StreetView from Google:


Here is an aerial photo of the location.  And here is a map link.