The three main feature to determine the cell would be:
1) If the Cell contains nucleus, then it would be Eukaryotic.
2) If the Genetic material is in Cytoplasm & can be seen at high wavelength (low intensity) then, it would be prokaryotic.
3) If cell contains extra complex organelles like Mitochondria, Chloroplasts, Golgi Apparatus, Lysosomes, then it would be eukaryotic otherwise it is prokaryotic.
Hope this helps!