A letter can be fake when the recipient realizes the sender is not actually the person who states it is, when the letter includes fake information about the sender or his/her address, and when its content is not related to real events or distortions the reality in favor of the sender's goal. Also, there can be grammatical incongruences between the age of the sender and the way he/she writes (the letter may be written by a kid in representation of an adult). Any of these characteristics can be present in a fake graduation letter.
