How can a timeseries data be declared as stationery?
Checks for Stationarity in timeseries data
 Look at Plots : plot a run sequence plot to see anything with an obvious trend or seasonal effects
 Summary Statistics : partition your data into intervals and check for obvious or significant differences in summary statistics

Statistical Test : use statistical tests if the expectations of stationarity are met or violated.
ref:https://medium.com/@kangeugine/timeseriescheckstationarity1bee9085da05
ref:https://towardsdatascience.com/detectingstationarityintimeseriesdatad29e0a21e638