Answer:
Throughout history, along with the development in microscopic technology, scientists were able to form the cell theory that: a) All cells come from pre-existing cells. b) All living organisms are composed up of one or more cells. c) Cells are the basic structure, function, and organization of all living organisms. Additionally, scientists, such as Louis Pasteur in the late 19th century, disproved the theory of spontaneous generation. Now, if one does not believe in this theory, they could either use a microscope and observe cellular division or reproduction/growth of organisms, or review the research from Matthias Schleiden, Theodor Schwann, Rudolf Virchow, etc. With the modern developments in scientific technology, people can observe the cell theory at their fingertips.
(Remember, do not copy this answer, that is plagiarism! But, please use it to guide your thinking!)