How to check Inputs Fields blank or not in Gridview

protected void btnSubmit_Click(object sender, EventArgs e)
{
        foreach (GridViewRow gvr in gridEvalution.Rows)
        {
            RadioButtonList rbl = (RadioButtonList)gvr.FindControl("rbtnList");
            if (rbl.SelectedIndex == -1)
            {
                ScriptManager.RegisterStartupScript(this,this.GetType(),"Alert()","alert('Please Select All Question.');",true);
                return;
            }
        }
}