How can I identify contacts that are matched with contacts in LinkedIn?

Contacts that your organization matches with contacts in LinkedIn using the LinkedIn Sales Navigator include the value LINKEDIN in the Original System Reference for Party attribute. You can create a BI report using this attribute as a filter from the Sales - CRM Contacts subject area or from any other subject area with a Contacts folder (Contact > Contact - Additional Attributes > Original System Reference for Party)

You can also use a direct SQL to get the matched records:

Select ORIG_SYSTEM_REFERENCE,OWNER_TABLE_NAME,OWNER_TABLE_ID from fusion.hz_orig_sys_references 

Where ORIG_SYSTEM='LINKEDIN' OWNER_TABLE_ID is the record ID, OWNER_TABLE_NAME is the main table where the record is located (for example: HZ_PARTIES for contacts and accounts, MOO_OPTY for opportunities, and so on).