A "tall" brown or light-colored dorsal fin that is generally uniform in color belongs to a hammerhead shark, however there are other species of shark and guitarfish fins that also have "tall" fins. See the information below for additional details on how to distinguish hammerhead dorsals from other tall fins.
Distinguishing hammerhead dorsals from guitarfish and blacktip shark fins
Dorsal fins that are tall and slender and dull brown or light grey are probably one of three species of hammerhead sharks: great hammerhead (Sphyrna mokarran) scalloped hammerhead (S. lewini) or smooth hammerhead (S. zygaena).

Tall dorsal fins can also come from several species of guitarfish or blacktip sharks. In guitarfish first dorsal fins, cartilaginous blocks do not extend across the entire fin base (Image A). In hammerheads, these cartilaginous blocks are present along almost the entire fin base (Image A). Guitarfish dorsal fins also exhibit a glossy sheen (Image B), and some species also have white spots, unlike the dull brown, uniform coloration of hammerhead dorsal fins.

Some blacktip shark (Carcharhinus limbatus) first dorsal fins exhibit O-A/W that is close to or slightly greater than 2.5. However, they often (but not always) have a black spot on the dorsal fin apex, and the fin has a glossy appearance that is unlike the dull of the hammerheads (Image C).

Blacktip shark pectoral fins are also longer and more slender than the short, broad fins of the hammerheads (Image D).

Distinguishing hammerhead dorsals from mako and thresher shark fins
First dorsal fins from mako, thresher and hammerhead sharks are tall and slender from leading to trailing edge.  In the image below, thresher and mako fins (top row) are slate to dark grey in color.  Great hammerhead first dorsal fins (bottom row, left) have a distinctive curved shape and are a  much lighter grey than mako or thresher fins.  Scalloped and smooth hammerhead first dorsal fins (bottom row, right) are similar in shape to the dorsal fins of mako and thresher sharks, but they are much lighter in color and are usually light brown instead of grey.