Launch a AfterEffects rendered in background with cmd update
After Effects has a seperate executable for the render engine.
Have a look at this article from Adobe for some usage examples and an overview of the commands.

http://helpx.adobe.com/after-effects/using/automated-rendering-network-rendering.html

aerender -project c:\projects\project_1.aep -comp "Composition_1" -output c :\output\project_1\project_1.avi


As you define the output file yourself you already have the path available. As soon as the command finished you know your file is done rendering and you can process it further.