Mocha 'before each hook' message in red. How do I know what specifically is wrong?

I ran into this problem when in the beforeEach I accidentally called done() twice (I called it once at the end of the beforeEach, but also called it again via an async function called in the beforeEach).

When I ran the tests in watch mode I got the error message you described without any additional information; when I ran the tests normally I did not get any errors. I reported this on a related ticket.

How do I know what the error is?

Debug it just like you would any normal code. If you are making assertions inside a beforeEach callback, you are abusing the framework. Assertions belong in the it callbacks, so refactor that.

It's also probably not just forgetting to call done because mocha has a clear error message when that happens.

Thus your code is probably throwing an uncaught exception and you can use your favorite flavor of debugging to track it down. I like running mocha with --debug-brk and debugging with node-inspector, but some console.log statements should also suffice. Note passing only the relevant test file to mocha and using the describe.only or it.only techniques can keep the test suite small and focused while you track down the root cause.