"How can I tell whether the EPO has successfully received the priority documents from the USPTO?

As soon as the EPO has included in the file of a European patent application a copy of the previous US application, from which priority is claimed, it will inform the applicant of the European patent application accordingly.

In the event the EPO is unable to electronically retrieve the necessary documents, the EPO will contact the applicant with a request to supply the priority document via the conventional (e.g., paper) route.

USPTO FAQ - Questions and Answers (12FEB2007)