No one has witnessed their own death, however almost all of us are totally convinced that we will die someday. Without a direct experience of death, one can say that that death is only a belief. Of course we see many others who die every day, its all very common and obvious. What's not so obvious is that we see only their bodies dropping dead. The rest is mere extrapolation of same event in our own case.
Fantasies of Death
We see that a body is destroyed after death, and we actually ensure that it is destroyed by cremating it. So it is fair to conclude that our own body will be destroyed, and since our body is no different than all those dead bodies, there is no reason to conclude otherwise. Anyhow one cannot witness the destruction of one’s own body. It is just an assumption.

