Hi Nuno ,

The event handler for the ItemClick has ChartItemClickEventArgs which can give you information about the exact data point you have clicked and DataSeries to which the data point belongs.

You could check if the data series of the data point are the same as those at index x in radChart.DefaultView.ChartArea.DataSeries collection. Like this: 
Or if that is not enough (or you are using item mappings) you could set each data series a different LegendLabel and distinguish the data series by it:
I hope this helps.
 
Regards,
Petar Kirov
the Telerik team