The simplest way to distinguish a prokaryotic cell from a eukaryotic cell is to check for
Nucleus
Of the choices given, the most reliable way to tell the difference between cell types is by checking the presence of a well-defined nucleus, which is found only in eukaryotes. 'Eu' means 'true' and 'Karya' means nucleus. Both prokaryotes and eukaryotes have the plasma membrane, DNA, and proteins.