I want to delete connections in tnsnames.ora. I need to figure out which ones are currently in use, so that I don't delete them mistakenly
I don't think you can get that from the database server side. The tnsnames.ora file is local to the client, so you could have a net service name (connection name) of 'ORANGE' that connects to a database service called 'BANANA'. 

I had a look at some of our listener log files, and they don't have this info. The SYS_CONTEXT function exposes a lot of info but not the client's local net service name.  https://docs.oracle.com/en/database/oracle/oracle-database/12.2/sqlrf/SYS_CONTEXT.html#GUID-B9934A5D-D97B-4E51-B01B-80C76A5BD086

If you want to differentiate connections, you could have multiple service names for the database, e.g. 'DB1_OLTP', 'DB1_REPORTS', 'DB1_NORMAL'. The service name can be used to assign the session to a resource consumer group to limit CPU etc.