What is the property to call the value of an item selected from a list?

I had a question while developing this code, I made a drop-down list in which each one has the value of an ID.

Now I want to develop a condition to disable more drop-down lists and enable them until you select an item where its value ID > 0 .

My problem is that I do not know what property I need to call this item to check its value when they select it, while I was investigating I got involved.

//un poco de idea de como esta desarrollado mi Dropdownlist

private void Lista1()

{
   //codigo resumido como el de matrix

    string selectSQL = "SELECT ID, Nombre FROM Muestra";

            ListItem newItem = new ListItem();
            newItem.Text = reader["Nombre"] + "";
            newItem.Value = reader["ID"].ToString();
            listauno.Items.Add(newItem);
}

//ejemplo de la condicion a formar

private void Habilitar()

{

    if (condicion si item seleccionado es mayor al id 0 de la tabla Muestra))

    {
     //habilitar listas
        listados.Enabled = true;
        llstatres.Enabled = true;
    }
    else 

    {
         //desabilitar

        listados.Enabled = false;
        listatres.Enabled = false;
    }


}