Adjusted .nuspec file, added property to interface as well.

Build/MQTTnet.nuspec

@@ -22,8 +22,9 @@
 * [Server] Added interceptor for unsubscriptions.
 * [MQTTnet.Server] Added interceptor for unsubscriptions.
 * [MQTTnet.AspNetCore] improved compatibility with AspNetCore 3.1
+* [MQTTnet.Server] Added option to check whether the server is already started properly or not.
     </releaseNotes>
-    <copyright>Copyright Christian Kratky 2016-2019</copyright>
+    <copyright>Copyright Christian Kratky 2016-2020</copyright>
     <tags>MQTT Message Queue Telemetry Transport MQTTClient MQTTServer Server MQTTBroker Broker NETStandard IoT InternetOfThings Messaging Hardware Arduino Sensor Actuator M2M ESP Smart Home Cities Automation Xamarin</tags> 
     <dependencies>
       <group targetFramework="netstandard1.3">

Source/MQTTnet/Server/IMqttServer.cs

@@ -27,5 +27,7 @@ namespace MQTTnet.Server
 
         Task StartAsync(IMqttServerOptions options);
         Task StopAsync();
+
+        bool IsStarted { get; }
     }
 }