Hi Jonathan,
The Method autofill actions when creating a new lead or a new customer can be found on the lose focus events of the fields. For example; there are actions on the lose focus event of the Company Name field on the New Customer screen that copy the data into Bill To line 1. You should be able to locate all of these autofills by inserting some dummy data into these screens and taking note of what field lost focus when data was filled in automatically.
Another thing to keep in mind is that your browser may attempt to automatically fill in data as well. In order to eliminate this possibility, you can clear your browser’s autofill form data. I hope this helps.
-Audisho