According to 14 CFR 61.51(h) Logging training time.....
1. A person may log training time when that person receives training from an authorized instructor in an aircraft , full flight simulator, flight training device, or aviation training device.
2. The training time must be logged in a logbook and must:
(i) Be endorsed in a legible manner by the authorized instructor; and
(ii) Include a description of the training given, the length of the training lesson, and the authorized instructor's signature, certificate number, and certification expiration date.
Oddly enough, I have had several applicants show up for check rides missing the items described in 2 (ii). It is part of the DPEs job to insure the applicant meets the training requirements of Part 61. Without a legible description of the training given along with the CFI signature, cert number and expiration date, they can't verify the training has been done. The DPE can't do the check ride until this is fixed.