Yevamot 120 - When to testify about the husband's death
If witnesses saw a man die, and they knew and recognized him, then they can testify that he died, for the benefit of his wife, to permit her to re-marry. They must, however, be certain that it is indeed the right man. Therefore, they only do it if they saw the dead man's head with the nose, although the eyes and the mouth need not be intact. They cannot make an identification based on the man's body or his garments.
Furthermore, they must be sure that he indeed died. For example, if they saw him suffer from a deadly wound, hanging from the gallows or being devoured by a wild beast, they may not testify. He might have received medical help and recovered, the rope might have been cut, or the beast might stop before reaching a vital organ.
As a rule, they may also not testify if the man died more than three days ago - although there are exceptions where he can be recognized long afterwards.
A story to illustrate that the face and the nose are crucial for recognition. Abba bar Marta owed money to the family of the Exilarch. He took wax, attached it to a rag and from there put it on his forehead. He then passed in front of his creditors, and they did not recognized him.
Art: Landscapes with Wild Beasts by Roelandt Jacobsz Savery