The effects both events have on people's lives is one of the reasons why it is so important to monitor and study the causes of ENSO activity.
Generally El Niño is associated with drought events especially in the tropical rainforests, but floods might happen as well in other parts of the world. These effects such as droughts and floods caused by a particular phenomenon in a remote locality away from its center of activity is called Teleconnections by the climate scientists.
One way to do so is to regularly look at the state of the sea surface temperatures in a few
key regions across the tropical Pacific. The most important ones are the Niño 3 and Niño 4, located in the eastern and central parts of the basin, and basically when the SSTA in these regions exceed the +- 0.5 degrees threshold for a few months (including the winter, which is the peak season of ENSO activity) you can say there's an El Niño or La Niña happening.
Another way to look at ENSO activity is through the Oceanic Niño Index (ONI) and the Trans-Niño Index (TNI), which also take into account the temperature through the Pacific key regions, and the Southern Oscillation Index (SOI) for the sea level pressure and the atmospheric part of ENSO. All of these time series are available for download at the
NOAA website.