How can we determine whether a time series data is stationary and which tests can be used to check for stationarity?

To determine if a time series is stationary, we check if its statistical properties like mean, variance, and autocorrelation are constant over time. Common tests for stationarity include the Augmented Dickey-Fuller (ADF) test, the Kwiatkowski-Phillips-Schmidt-Shin (KPSS) test, and the Phillips-Perron (PP) test.