The most likely structure that would indicate that a cell is a prokaryote is the absence of a nucleus. Prokaryotic cells are characterized by their lack of a membrane-bound nucleus or other organelles, and their genetic material is contained within a single circular molecule of DNA. This is in contrast to eukaryotic cells, which have a true nucleus and other membrane-bound organelles. Therefore, if a cell is observed to lack a nucleus, it is most likely a prokaryotic cell.
~~~Harsha~~~