To retrospectively figure it out, as I wrote earlier:
To get a feel on whether a 90% fill factor is adequate for your index, you can monitor
sys.dm_db_index_physical_stats.avg_page_space_used_in_percent over a period of time.
If you find that it quickly and significantly deviates from the 90%-space-used value enforced originally by the fillfactor setting, then that means the 90% fillfactor value is probably too high.
SELECT avg_page_space_used_in_percent
FROM sys.dm_db_index_physical_stats(@db_id, @object_id, @index_id, NULL , 'DETAILED');
Whether or not page splits are occurring is not as much the issue, as whether they are occurring at a statistically significant rate, and if that is the case, you will need to lower your fill factor.
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