There are several ways to judge whether there is Monopterus albus in the river ditch: observing the Monopterus albus hole, observing the foam near the water, observing the characteristics of the hole and so on.

1. Observe the eel cave: generally, the eel cave is in the water or on the shore, not in a deep place. There will be a deep feeling from above, and you won't see it all at once. You can feel it with your hands. It's very cold and deep in the eel cave. If all the requirements are met, it is probably an eel hole.

2. Observe the foaming at the water's edge: In summer and autumn, if you see a layer of foaming at the water's edge, it will be very abrupt. If it's not water pollution, it's probably next to an eel cave, and it's an eel that has been pregnant for a while and even bred.

3. Characteristics of observation hole: The eel hole can be identified by the roundness and smoothness of the hole, which is located in the area 8 cm above the waterline. After discovering the cave, we should also see if there are any traces of crawling from the hole to the waterline. Sometimes the hole of Monopterus albus will be below the waterline, which is mostly because the water rises and it is too late to adjust the hole.