How do I know I've hit the threads limit defined in Node?

Are you looking implementation in nan or js?

In Nan impl:

You have to do it manually. Maintain a map where key is int and value as workAsyn. Push at every call and delete when workAsyn complete. Do this for every request.

Compare the size of map with your thread limit defined.

I analyzed nan and libuv sources. Unfortunately, now there's no way to get the number of used threads. If only to add this feature yourself.