There are several methods for determining which 5.3 engine you have. The most trustworthy method is to count the teeth on the reluctor. In general, a 24-tooth reluctor indicates a Gen III engine from 2004 or before. Nevertheless, there are exceptions: The 6.0L LS2 engines found in 2005 Corvettes, 2005-2006 GTOs, and 2005-2006 Trailblazer SS models all include a 24-tooth reluctor, however they are members of the Gen IV engine family. To check this, you should examine the rods that are fully floating.
Once you have narrowed down your engine to either Gen III or Gen IV, you can use other distinguishing characteristics to differentiate them. Gen III engines typically have pistons with a flat top, whereas Gen IV engines have pistons with a dished top. Additionally, the valve cover of Gen III engines is thicker than that of Gen IV engines.
Another way to determine this is by examining the oil pan. Gen III engines have shallow oil pans, but Gen IV engines have deep oil pans. Gen III engines have a 2-bolt crankshaft flange, but Gen IV engines have a 3- or 4-bolt crankshaft flange.
In conclusion, if your engine has a 24-tooth reluctor, it is likely a Gen III or Gen IV. Note the full-floating rods, piston design, valve cover thickness, oil pan depth, and crankshaft flange size to be certain. This information should allow you to determine which 5.3 engine you have.