If you want to get to know how your users are using OWA to connect to mailboxes you can use following two solutions.
In any environment where Microsoft Exchange OWA service is enabled for user mailboxes, we can check how many users actually use OWA access through their mailboxes.
The first solution presents how to check the number of current connections to the mailbox using OWA.
For this purpose, we need to run this script by calling the Get-CASActiveUsers for all servers that provide OWA connections, so mostly for all CAS servers.
Following script will list all current connections to OWA and RPC on CAS servers:
English 
polski