FCL 740 says that you must train for at least one hour. Who will decide if the adequate training is one hour or more than that? The FI.
If you sign the licence as the FI who has trained the pilot, you implicitly but necessarily endorse the fact that the training was adequate.
If you sign the licence as a FE who has not trained the student, you just check that the logbook shows at least one hour of training etc., there is no other responsibility with you.