Demo Server and Tests
You can try IdentityServer4 with your favourite client library. We have a test instance at demo.identityserver.io. On the main page you can find instructions on how to configure your client and how to call an API.
Furthermore we have a repo that exercises a variety of IdentityServer and Web API combinations (IdentityServer 3 and 4, ASP.NET Core and Katana). We use this test harness to make sure all permutations work. You can test it yourself by cloning this repo.