Some of the migration and housekeeping tools have some reporting on this, but if you've not got (or don't want to buy) one of them, then plain Jira has no direct way to do this.
There is a slogging way to do it -
If you're an admin you can look at the list of fields, but there isn't any why I'm aware of to export the list of just the ones you are actually using. Depending on how you've managed Jira, every custom fields should be used in at least one project. If that's true reviewing the system fields shouldn't take long.
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