The easiest and most dangerous way is to just stick your hand in and see if you get a slight shock or tingly sensation. This is not a recommended method to test for stray current.
All kidding aside, the best way to check for stray current is to get a multi-meter. Set the meter to 120 AC Voltage. Place the tip of the black prong into a grounding hole of an electrical outlet. Now insert the tip of the red probe into the aquarium water. Watch the meter to see what the voltage reads. If it's anything above a ZERO reading, you have stray voltage in your water.
If there is stray voltage, unplug each heater and pump that is submersed in the water, I would start with the new heater. Then plug them in one at a time and read the multi-meter each time another item is plugged in. Once you see the voltage increase, you have found your culprit.
Looking at your pic though, the anemone does appear to be bleached (may just be the blue hue from the lights) and as mentioned earlier, a better pic with the blue turned down and if possible, a little closer, would help.
