Yevamos, Chapter 16, Mishnah 3. This mishnah opens a discussion that in fact continues until the end of the tractate, dealing with the details of establishing that a particular person has died.
"Ein me'idin ela al partzuf hapanim im hachotem":
One may not testify to a person's identity, to the identity of a body, unless the facial features were identified together with the nose.
What is included in "the facial features"? This is a discussion in its own right: some understand that it refers to the cheeks, and the Gemara cites a beraisa that includes the forehead as well; some include the eyes and some do not. In any case, part of the face is required together with the nose.
"Af al pi sheyesh simanim begufo uvechelav":
Even when there are identifying marks on his body and on his garments, recognition of that part of the face is still required. The Gemara distinguishes between two types of identifying marks:
Ordinary marks - such as tall or short, dark or light skinned. These are not specific and could be shared with other people.
Conclusive marks - certain identifying signs, unique to one individual alone.
The ruling that identifying marks do not help and that facial identification is required applies to marks that are not conclusive and not unambiguous: short, tall, dark complexioned and the like. But where the marks are conclusive and unambiguous, facial identification is not needed.
However, regarding "chelav" - his garments - even conclusive, unambiguous and unusual marks are not sufficient. The reason is that the garments may have been passed from one person to another, and therefore they are no proof, unless there is identification of the face.
"Ein me'idin ela ad shetetze nafsho":
Witnesses cannot testify that a person has died until they actually saw with their own eyes his soul depart. Even in the following situations one may not testify:
"Va'afilu ra'uhu meguyad" - cut to pieces.
"Vetzaluv" - that they saw him hanging on the cross.
"Vehachayah ochelet bo" - that wild animals were devouring him.
In all of these the matter is not certain, since it is possible that the person survived all of this.
"Ein me'idin ela ad sheloshim yom":
The Talmudic explanation here establishes that one may testify only within three days of the death, because after that the process of decomposition begins, and the body can no longer be identified with certainty.
Rabbi Yehudah ben Bava disagrees and holds that no absolute, precise time line can be set: "Lo kol ha'adam velo kol hamekomos velo kol hasha'os shavin":
"Lo kol ha'adam" - some decompose quickly and some more slowly, all depending on the type of body.
"Velo kol hamekomos" - a hot or cold climate, humid or dry, affects the process.
"Velo kol hasha'os" - a different time of year or of day may also have an effect.
In summary: In this mishnah we learned that testimony about a death requires identification of the facial features together with the nose; that marks on the body which are not conclusive are insufficient, while regarding garments even conclusive marks do not help, out of concern that they were exchanged; that one may not testify until the soul has actually departed, even in cases of a body cut to pieces, crucifixion, or being devoured by an animal; and that the Tannaim disagree whether a fixed time frame for identification can be set, or whether it depends on the person, the place and the time.