How can scientists tell if the cells aren’t alive anymore?

There are many ways that a scientist can tell if cells aren’t alive anymore.

One example of this is by figuring out if a cells membrane has been damaged- cells with damaged membranes are usually considered dead because the membrane is key for the cells survival.

They usually find this out by using a cell-impermeant DNA binding dye like propidium iodide. If the membrane is intact then the dye can’t enter the cell because the membrane will act like a barrier. A damaged membrane will allow the dye to get through, proving that the cell is dead.

Hope this helps to some degree :)

Looking to see if the cell membrane is damaged or using a cell impermeable DNA binding dye

The answer is keep records