Stress test / hang test of large Node.JS application

We have a large website with express and the whole application is written in Node.JS.

When the mongo db is down our app starts hanging when you refresh the browser on home page and also in other page. This is caused because of the behaviour of the mongoose library when the mongo database goes down. the app is served from ubuntu 14.04 machines.

My question is how to test application "hangs".

What kind of tests should I perform to know that i don't have any more web app hangs. The problem is that a hang can be caused by multiple reasons and not just by the mongoose issue which i have fixed.
How can I test that? What kind of tools and techniques are recommended?