how do I know how much poly does my blender models have?

Look at the top of the blender window to show number of vertices and faces

Resource Information: Gives you information about application and system resources. It tells you how much memory is being consumed based on the number of vertices, faces and objects in the selected scene. It is a nice visual check to see if you are pushing the limits of your machine.

look at the top of your window

the number after Ve: is number of vertices
the number after Fa: is number of faces

:edit: nevermind richard beat me to it