Question

when will you suppose that the cell is dead

Answer 1

You know its dead when its done apoptosis and its not functioning.

Answer 2

The main sign of life in the biological cells,is their continuous biological works and activities.An alive biological cell continuously performs it's biological activities.

But,when there is no trace of biological activities in a cell and the cell starts drying out.

Then we can easily consider that the cell is dead or in the near stage of biological death.