I read this (below) in a chapter (The Portrait of a Lady):
When I decided to go abroad for further studies, I was sure my grandmother would be upset. I would be away for five years, and at her age one could never tell. But my grandmother could.
What exactly does tell imply here? Has it been used with reference to death?
"Tell" here means "know or be certain". The writer is saying that, considering his grandmother's age, he could not be certain that he would see her when he returned after five years, because she might die during that time. The final words "but my grandmother could" imply that his grandmother had no such uncertainty, and in the following paragraph the writer says: "After five years I came back home and was met by her at the station". The writer is, perhaps, subtly conveying the arrogance of youth, because the young man ignores the fact that he himself could not be sure of being alive in five years' time.
The expression "can/could never tell" is broadly the same as "you never know", meaning that one cannot predict something. Exactly what it's saying that "one could never tell" s unclear from the quoted text.
This usage of tell can also be seen in phrases such as "I can tell that you're upset". That means "I am able to perceive that you are upset".
