Validate Check box in grid view based on drop down selected Item

Hi,
I have a requirement were in I need to select a drop down to choose an action. This action has to be performed on the rows (where in the check box is selected)in the grid . Now how do we validate / check any of the checkbox in the grid is selected? I guess there is no such validation. How do we perform such validation
Thanks and Regards.
N1ZAM