Patterns in ocean temperature precede el Niño and la Niña events months in advance, so by seeing what the ocean temperature is doing especially in the western Pacific, we know whether we will have an el Niño or a la Niña.

Here is an important and interesting site where you can read that by looking at the sea surface temperatures (SST) and air circulation in the atmosphere at the equator, El Niño o La Nina can be predicted:
NOAA .

They think we probably won't have an El Niño or La Niña this spring or summer [2020].

Predictions of El Nino and La Nina events are made using climate models . At a very high level , when equations are given a bunch of numbers describing current conditions and produce future conditions like those of past El Ninos/La Ninas, scientists feel confident in saying that they expect such an even in the near future.

These models take in readings from observation sensors located throughout the Pacific Ocean. Some of the variables include water and air temperatures, wind speeds and directions, and air humidity.