1. All living things are made up of cells.

2. Cells are the basic unit of structure and function in all living organisms.

3. All cells arise from pre-existing cells.

Scientists can use this theory to determine if an object is an organism by looking for the following:

* Presence of cells: The most crucial element. If an object doesn't contain cells, it cannot be considered alive according to cell theory. This can be determined through microscopy or other biological tests.

* Cellular organization: Organisms exhibit a specific level of organization beyond just individual cells. They may have tissues, organs, or even organ systems, all composed of cells.

* Metabolic activity: Living cells exhibit a wide range of metabolic activities, like respiration, energy production, and growth. Observing these processes within an object can be strong evidence of life.

* Reproduction: Organisms are capable of reproducing and passing on their genetic material, which is contained within their cells.

Examples:

* A rock: A rock is composed of minerals and does not contain cells. It shows no signs of metabolism or reproduction. Therefore, it's not considered an organism.

* A virus: Viruses are considered to be non-living entities because they lack the ability to reproduce independently and rely on a host cell's machinery for replication. Although they can evolve and contain genetic material, they do not possess the fundamental characteristics of a cell.

* A bacterium: Bacteria are single-celled organisms, fulfilling all the requirements of cell theory. They are self-sufficient, capable of metabolism, reproduction, and even adaptation to their environment.

Important Note: While cell theory is a valuable tool for classifying organisms, it's not a definitive answer in all cases. Some entities, like viruses, present challenges to the strict definition of "life". Further research and advancements in biology may refine our understanding of life and its defining characteristics.