Detecting TestServer in asp.net core?

I am using TestServer to instantiate my asp.net core web app during test.

var server = new TestServer(new WebHostBuilder().UseStartup<Startup>());

In the Startup of the web app, I access different resources, depending whether the environment is Production or Development. Now I need a third case: Test. Can I detect in the Startup, that it is being invoked by the tests via the TestServer?