There are two ways to determine the year your 604 was manufactured. The first way is to look at the numbers on the front of the block. These numbers are located on the Passenger side front, on a square pad on the deck surface. You'll want to look at the PO4 number. For instance a 2009 engine would say 9PO4, a 2006 engine would say 6P04, etc. 

However, the most accurate way is to look at the serial number on the block, which is located on the drivers side, rear, on the extended part of the deck surface that goes over the bell housing area. The date code for the engine is built into the serial number after the first two numbers. As an example, a 2013 engine would read something like this, 1013426. The “13" designates it as a 2013 engine. Here are some more example.........1011390 is a 2011 engine, 1110900 is a 2010 engine, 05747 is a 2007 engine, etc. 

GM has come full circle now on the PO4 numbers, so the first method alone is not entirely accurate. The first engines were stamped 4PO4, and the 2014 engines are stamped 4PO4. The difference in a 2004 engine and a 2014 engine serial number is as follows......Example: 2004 engine would be something like 06435, and a 2014 engine would be stamped T0614149.

Here's an example of the PO4 code on a 2007 engine:

604 Crate PO4 Code on Engine

And here is a shot of the serial number block on the same engine:

604 Crate Serial Number on Engine