Brake fluid normally has more of a thin glossily coat/look to it. Also you can look around to see if signs of where it might be leaking from. Have you replaced anything on your brakes? Have you tried to bleed them?
The Birf Soup you can usually see oozing from the back of the hub at the rubber wiper. You could just clean up the knuckle and replace the inner axle seal as long as your bearing and all the knuckle parts are still good.
