If youre looking for manual way to look it up rather than a programatic way...
Fastest way i've found is to just start typing it in the advanced issue search. When the autocomplete popup displays, it will list the field appended with "- [cf#####] the #### part is the field ID.
@Randy_Ho You've found the easiest process - just to outline it in 2020 on Jira Server:
1. go to Issues > Search,
I'm on Jira Server 8.5
If you are an admin, you can go Administration > Custom Field and hover over the Edit operation to find the id of the field in the url. Or right click and open the Edit url to open it in a new tab. That way you can see the field id in the browser.
You can also get the id from Database if you have access to the DB tables.
If none of that is an option, use the JIRA REST API to retrieve the fields available for a given project and issue type like this:
I have multiple projects that use variations of the same base workflow. The variations depend on the requirements of the project or issue type. The variations mostly come in the form of new statuses ...
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