How would a scientist have made the distinction between a unicellular and a multicellular organism in the 17th century (1600s), the mid-20th century (1900s), and in the 21st century

In the 17th century, the scientists would have used the ordinary microscope to distinguish between unicellular and multi cellular organisms. In the 1900s, the scientists would have use light microscope and during the 21st century, the best tools that are available for use are electron microscope and transmission electron microscope.