If you looked at unknown cells under a microscope, what could lead you to correctly conclude that they are prokaryotic cells

The main difference between a prokaryotic cell and a eukaryotic cell is that the prokaryotic cell have no true nucleus instead its genetic material is just suspended in its cytoplasm termed as nucleoid. The nucleus when viewed under the microscope is a membrane bound which is easily distingushable compared to the nucleoid.

Another distinguishing feature is that prokaryotic cells are usually smaller than eukaryotic cells.