C-sharp SQL Select Abfrage vergleichen

 int wnr = Convert.ToInt32(textBox1.Text);
            string rez = textBox2.Text;
            string kuehl = textBox3.Text;
            DateTime mydatetime = DateTime.Now;

            string abfrage = "INSERT into hefe (WNR,REZ,KUEHL,EINL) " + "VALUES (@WNR, @REZ, @KUEHL, @EINL)";  
            string updateabfrage = $"SELECT AUSL FROM hefe WHERE WNR = {wnr} and AUSL is null";  
           
            SqlConnection conn = new SqlConnection("Data Source=khdb\\sqlexpress;Initial Catalog=ProdHefe;Integrated Security=True");  
            SqlCommand insert = new SqlCommand(abfrage, conn);
            SqlCommand select = new SqlCommand(updateabfrage, conn);

            // Prüfung ob Wagen noch in Verwendung
            if (select.??????)
            {
                MessageBox.Show("Wagen noch in Verwendung", "Wagennummer", MessageBoxButtons.OK, MessageBoxIcon.Error);  
                System.Environment.Exit(0);
            }


            // SQL  Werte schreiben

            insert.Parameters.AddWithValue("@WNR", wnr);  
            insert.Parameters.AddWithValue("@REZ", rez);  
            insert.Parameters.AddWithValue("@KUEHL", kuehl);  
            insert.Parameters.AddWithValue("@EINL", mydatetime);  
            
            conn.Open();
            insert.ExecuteNonQuery();
            conn.Close();

            textBox1.Text = "";  
            textBox2.Text = "";  
            textBox3.Text = "";