Three steps to using this guide
Step 1.  Distinguish 1st dorsal fins from other highly-valued fins: pectoral fins and lower caudal fins (see below).
Step 2.  Look for white 1st dorsal fin markings, and use the online identification tool to identify either porbeagle or oceanic whitetip sharks or exclude many species with black fin markings.
Step 3.  Take several simple measurements to help identify hammerhead 1st dorsal fins, which are much taller than they are broad and are dull brown or grey.

Step 1: Distinguish 1st dorsal fins from pectoral fins and lower caudal lobes

a. Check the fin color on each side

Dorsal fins are the same color on both sides (see right and left side views below).  In contrast, pectoral fins are darker on the top side (dorsal view) and lighter underneath (ventral view); (see both views below).

b. Check the base of the fin

Dorsal fins (D) have a continuous row of closely spaced cartilaginous blocks running along almost the entire fin base.  When looking at a cross section of the base of a lower caudal lobe (LC1), there is typically only a yellow, "spongy" material called ceratotrichia, which is the valuable part of the lower caudal lobe.  In some lower caudal lobes (LC2) there may be a small number of the cartilaginous blocks, but they are usually widely spaced and/or occur only along part of the fin base.  Usually, the lower caudal lobe has been cut along its entire base when removed from the shark; in contrast, dorsal fins frequently have a free rear tip that is fully intact.
Is the fin generally uniform in color?