NServiceBus 6 feature. When enabled in an endpoint this feature will report all event types that can be handled to the console at startup.

PublishHandledEventsAtStartup.cs

class PublishHandledEventsAtStartup : Feature
{
	protected override void Setup(FeatureConfigurationContext context)
	{
		var conventions = context.Settings.Get<Conventions>();

		context.RegisterStartupTask(b =>
		{
			var handlerRegistry = b.Build<MessageHandlerRegistry>();
			return new PublishedHandledEventsToConsole(handlerRegistry.GetMessageTypes().Where(conventions.IsEventType));
		});
	}

	class PublishedHandledEventsToConsole : FeatureStartupTask
	{
		private readonly IEnumerable<Type> messageTypesHandled;

		public PublishedHandledEventsToConsole(IEnumerable<Type> messageTypesHandled)
		{
			this.messageTypesHandled = messageTypesHandled;
		}

		protected override Task OnStart(IMessageSession session)
		{
			foreach (var messageTypeHandled in messageTypesHandled)
			{
				Console.WriteLine(messageTypeHandled);
			}
			return Task.CompletedTask;
		}

		protected override Task OnStop(IMessageSession session) => Task.CompletedTask;
	}
}

To enable in an endpoint, call endpointConfig.EnableFeature<PublishHandledEventsAtStartup>();