The best way would be to measure the draw, but doing that can trash a regular multi-meter fairly easily. A less precise, but safer way would be to just run a test of the fan powered directly from the battery. Attach a 30A fuse to a wire (16 gauge house wire, etc), and connect that directly from B+ to pin #2 of the fan. Then connect another wire from battery negative to pin #1 of the fan. If the fan itself is the problem, the fuse should blow fairly quickly.