how to find which cell is clicked in a gridview in asp.net ?

By default, GridView does not support cell click event handling but you can add that feature.

Have a look at this article: Extended ASP.NET GridView with cell click events[^]

In case you just need to do some small things for the control click in a particular cell, try:

protected void GridView1_RowDataBound(object sender, GridViewRowEventArgs e)
{
 if ( e.Row.RowType == DataControlRowType.DataRow )
  // for example, you got linkbutton in that cell
  LinkButton linkButton = (LinkButton)e.Row.FindControl("LinkButton1");

  if (linkButton != null)
  {
    // do your stuff
    //linkButton.Attributes.Add("onclick", "window.open('someURL'); return false;");
  }
 }