List of Indexed Field in Salesforce

     Indexed fields are used to build a SOQL queries in more efficient way. Here is the list of indexed fields in salesforce.

The following fields are indexed by default.

1. Primary keys (Id, Name, and Owner fields)
2. Foreign keys (lookup or master-detail relationship fields)
3. Audit dates (such as LastModifiedDate)
4. Custom fields that are marked as External ID or Unique

You can also find the Indexed Field from the list of field option. 

You can also checkout the usage of index fields from the below link,

https://help.salesforce.com/apex/HTViewSolution?urlname=Custom-indexes-for-an-organization-to-help-improve-performance-1327109991720&language=en_US