looking at a cell under a microscope you note that it is prokaryote how do you know

Question

setha · Answer

prokaryotic is a microscopic single-celled organism that has neither a distinct nucleus with a membrane nor other specialized organelles. 
So by it not having a nucleus and specialized organelles 

Hope this helps, if it does please medal then close the question

mathmate · Answer

It would also inspiring to read the following: http://www.majordifferences.com/2013/08/difference-between-prokaryotic-cell-and_27.html

mangotangochick · Answer

The main difference between a eukaryote and a prokaryote would be in eukaryotes, you would find a cell nucleus, enclosed in a nuclear envelope. This nucleus contains chromatin,  which is DNA wrapped around histone proteins.

In prokaryotes, however, the genetic material is not contained, but is free-floating in the nucleoid (a region in the cytoplasm where DNA is found) and is sometimes called 'naked' DNA, because it is not wrapped around histone protein. It is also circular DNA, whilst you find linear DNA in eukaryotes. 

There are also other differences such as:

Prokaryotes do not have mitochondria
They have 70s ribosomes instead of 80s ribosomes
No membrane-bound organelles in the cytoplasm

Please also read the article posted above if you would like further explanation on this ^^