what would be the best way to determine is a field on a form is being used by a script?

The best recommendation is to download the entire SuiteScripts folder from the File Cabinet as a .zip file, unzip the contents, then use a program like Agent Ransack or similar file content searching tool to look for the field’s script id in the contents of any script files. Keep in mind, it could still be referenced by external integrations through SuiteTalk or SuiteAnalytics Connect, by saved searches or workbooks, or in PDF templates, too

There are some bundles that do this as well like strongpoint and probably some others.

We used to use Strongpoint, and it does have this feature. It’s pricy to purchase if you just need to do this one time, but if this is a frequent need, then it could be worth it. Also, depending upon how much data is in your account, it could take several days for Strongpoint to “spider” your system after the initial install before you’d be able to get the answer you need.