If you happen to spot debris like seaweed, an area of water that is ruffled that is moving away from the shoreline towards a certain region, it's an indication of danger. A dangerous rip-current is likely to originate from below the ocean's surface. You can also tell that there is a gap between waves or the discolored water. It's extremely dangerous, so if you happen to be swimming during this time, you should proceed to swim parallel to the shore and away from the current.