The absence of a nucleus.

Here's why:

* Prokaryotic cells are simple cells that lack a membrane-bound nucleus and other membrane-bound organelles. Their DNA is located in a region called the nucleoid, but it's not enclosed within a membrane.

* Eukaryotic cells are more complex and have a true nucleus containing their DNA, as well as other membrane-bound organelles like mitochondria, Golgi apparatus, and endoplasmic reticulum.

Other observations that could suggest a prokaryotic cell include:

* Small size: Prokaryotes are generally much smaller than eukaryotic cells.

* Lack of internal membrane systems: Prokaryotes lack the complex internal membrane systems found in eukaryotic cells.

* Presence of a cell wall: While some eukaryotes have cell walls, they are more common in prokaryotes.

* Circular DNA: Prokaryotes have a single, circular chromosome, whereas eukaryotic cells have multiple linear chromosomes.

However, the absence of a nucleus is the most defining characteristic of prokaryotes and the most reliable indicator when observing a cell.