Tangential but related: can anyone point me towards what tools are being used to identify this kind of malicious JS?

I host a modest pile of WordPress and other applications, and all traffic gets proxied by nginx. I've been occasionally sniffing around for something that I could use to monitor the JS of outbound traffic and give me a heads-up if it smells like one of the sites is delivering something it shouldn't be, but I haven't found a tool for this that I like yet.