Here’s a straightforward way to do it:

1. Create and Open the Connection
   Use a SqlConnection with your failover partner in the connection string, then open it:

   using (var connection = new SqlConnection("Server=MyPrimaryServer;Failover Partner=MyMirrorServer;Database=MyDB;Integrated Security=true"))
   {
       connection.Open();
   
       // 2. Check which server you're connected to
       string currentServer = connection.DataSource;
       Console.WriteLine("Current Server: " + currentServer);
   
       // 3. (Optional) Run a simple SQL query to confirm
       var cmd = new SqlCommand("SELECT CONVERT(sysname, SERVERPROPERTY('ServerName'))", connection);
       string serverName = (string)cmd.ExecuteScalar();
       Console.WriteLine("Current Server (From SQL): " + serverName);
   }
   

• connection.DataSource gives you the actual data source (server) your app is currently connected to.
• SQL Query Method (optional): You can use SERVERPROPERTY('ServerName') to confirm the exact instance name.

This way, you’ll always know whether you’re on the primary or the mirror server at runtime, which is especially useful after a failover.