How to identify seasonal time series?

I am having sales volume data for 100 stock-keeping units (SKUs). Is there a statistical way of identifying seasonal time series?

• Perhaps it's too obvious to mention but I'd always start with plotting data against time of year (for the usual meaning of seasonality), and time of month, time of week, time of day as suitable. Telling us about your time unit (sales are measured hourly, daily, weekly or what?) and about you want to do with your data would allow expanded answers. – Nick Cox Feb 7 '18 at 10:26

Look at ets in the forecast package for R. While you are there, look at the seasonplot command to visualize seasonality.