How do I check for a clash

private void btnAdd_Click(object sender, EventArgs e)
       {

           bool isClashing = CheckClash();

           if (isClashing == false)
           {
               saveTimetable();

           }

           else
           {
               MessageBox.Show("The timeslot chosen is allready occupied!!");
           }
private Boolean CheckClash()
       {

           return false;
       }