CRO Billing can show you what invoices have been sent to QuickBooks.
The Invoice Statement report includes a "Sent to QB" field that shows which invoices have been synced to QuickBooks Online.
This report does not show the identity of the user who sent the invoices; however, an audit trail feature is currently under development, so stay tuned.
You can export this as a CSV to see the sync status for all your invoices.