how to automatically test my webpack-bundled web against FUOC?

I've built a React + Redux web which gets bundled with webpack. Due to a bundling error my web started to show FUOC behaviour (some components didn't inject their CSS to the server response, so there was a 'flash' of unstyled parts of the application before the final CSS was loaded). Other errors included FUOC for 3rd party components (which required manual work to do).

I have automated test for the server and different parts of my web. But how do I automatically test for FOUC (with my node.js toolchain)? I tought about selenium and phantomjs, but this seems like an overkill, and I still don't know how I could detect it.