Hi ScottT,
You can go to a particular subscription in Plesk UI (Subscriptions > subscription name) and see whether WPB is used or not under Other Resources tab, right column, "Sites published with Web Presence Builder".
Alternatively, you can go to Statistics (located in Tools & Utilities in 10.x up to 10.3.x, Tools & Settings in 10.4), click Summary Report, then select a Full Report or create a custom report with domains only (Full report on domains required).
We understand that both of these options might not be convenient in certain cases -- so please let me know what would be most convenient in your case. Thank you.