How does the USPTO determine if an applicant is entitled to the benefit of the foreign filing date?

The examiner generally does not examine the certified copy of the foreign priority application except to see that it contains no obvious formal defects and corresponds to the U.S. application. The examiner will consider the merits of the priority claim if:

• An intervening reference is found with an effective date between the foreign filing date and U.S. filing date
• Determining if a reference is prior art under the grace period
• An interference situation is under consideration

If the examiner finds the applicant is not entitled to the priority date, a rejection is made and the applicant must then perfect the claim. [MPEP 216]