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 :)
