How to make sure that Invoices went through?

I'm not sure if this is the right place to ask this question about EDI/IDocs.

I want to make sure that the invoices for a particular day were received by the customer. How can I do this at the SAP level?

I checked in WE02 and it shows me quite a few IDocs with status "07 Error during syntax check". Others are "53 Application document posted". But the customer complains that they have not been getting any invoices. Also, is there a way to see which IDoc number belongs to which Invoice number?

Please suggest.

Thanks