There is not currently an easy way to determine which connections are using SSL/TLS. The Performance Schema implementation from MySQL 5.7 added the CONNECTION_TYPE column to the threads table, which does specify whether a given connection is using SSL/TLS:
However, that implementation has not yet been merged into MariaDB. That task is being tracked as MDEV-6114, and it is currently stalled.
If we do not end up merging performance schema from MySQL 5.7, would it make sense to add this information elsewhere? It seems to me that this information would fit nicely in "SHOW FULL PROCESSLIST" output or the information_schema.processlist table.