How can we tell if a non-recurring line item has been invoiced?

There exists a field in the non-Recurring Invoice line item labeled with the API Name fw1__Related_Invoice__c that is filled in once it is invoiced. If the field is not currently visible, it may be necessary to make it visible in your page layout. The field shows the URL of the Invoice generated. Non-recurring line items are invoiced only once after they are added to the Recurring Invoice.