You can click the fields in the CRM form and check if there is an OnChange event.
It should be noted this isn’t a new problem for CRM 2015/2013 , there have been lots of tools created to help the process.
Here is my blog post on CRM form performance
CRM 2011/2013 Investigating CRM Form Performance issues
Things to consider
I used Fiddler to track what calls were being made out of the form
Script finder tool
I knew there was a script finder tool but I couldn’t find it when searching the internet. Thankfully a reader added a comment and of course it’s in the everyone’s favourite CRM tool or collection of Tools the XRMToolbox.
I did try doing a find in files but this proved to be very slow and seemed difficult, I eventually decided this process wasn’t really working.
So I looked at the problem a different way
How do I find what files are used when the CRM form is being used?
I came up with two main methods.
Why did this happen
You never want to duplicate code because if there is a bug in the code then you would have to fix all the various copies of the code and the chances are you would forget to change some of them. Later it would be difficult for the CRM developer to know which code was fixed and which code still had the bug in.