How to extract rainfall data from a spatial map for a particular Grid?

Hi,

I have generated the spatial map for rainfall. The area of study is large and I need to extract the data for a particular grid cell to validate the data with original data set. I have a complete dataset to plot this map but don't know which column or rows has which grid data. How could I do that? My figure is below and I need the data for a particular grid box marked with red color.