A few different queries / methods to list all fields for indexes.
index=yourindex| fieldsummary | table field
or
index=yourindex | stats values(*) AS * | transpose | table column | rename column AS Fieldnames
or
index=yourindex | stats dc() as * | transpose
or ;-)
index=yourindex | table *
index=your_index
| fieldsummary
| fields field values
It may just be on my instance but when using the fieldsummary command, the distinct count of values returned truncates to 500. There may be a limit configuration adjustment somewhere that would accommodate this.