There are multiple ways to check this:
1. Check the Inventory Service plugin logs from the master transmitter hosting these plugin's. The active plugins will be receiving scans from the workstations and inserting them into the Inventory database, and such activities would be logged in the Inventory Plugin logs.
2. Run a query on your database and check the count of Inventory Service channels on the workstations.
3. Check the inventory service URL on a random set of workstations in production environment.
Just to add to Adil’s suggestion, you can also check the access log of the master/mirror to see if there’s a client accessing this plugin (search for POST request against the plugin URL). This might be the easiest without having to check all inventory plugin logs (labeled by number for each URL).