In orgs with large data volumes, it sometimes becomes necessary to ask Salesforce to create a custom index on a particular field, or combination of fields.
Once Salesforce creates a custom index, is there any way for a System Administrator to see that it, or any other custom index actually exists? It seems like the only way to do this right now is to contact Salesforce Support and ask them.
Please Note: I'm wondering specifically about custom indexes, which are only created by opening a case with Salesforce, and not default indexes. This question, "What standard and custom fields are indexed?" does a great job listing fields that get indexed by default.
Best Answer
Sadly, as of August 2014, the only way to find out if a custom index exists is to open a case with Salesforce Support and ask them whether or not a specific field is indexed.
Apparently, the Setup UI of the support reps has a "Column Indexed" checkbox on their version of a Field detail page. This is how they are able to check if a particular field has an index. Makes me wonder what a two column index looks like on their screen...
Update: This feature has now gone GA. The Fields setup pages for Standard Objects, and the main setup pages for Custom Objects now display an "Indexed" checkbox for all standard and custom fields.