Debugging WebApi2 Login (/Token) endpoint

Using .NET WebApi2: I am getting an 500 internal server error when attempting to log in to a WebApi, at the /Token endpoint. I know there are some configuration parameters in Startup.Auth, but is there anyway I can set a breakpoint, or otherwise see what is showing up on the server? (say, before the authentication check happens, and after -- not looking for a Fiddler type intercept solution).