how to define detection rule for a package in sccm 2012

I know I can define detection rules for any application deploy in sccm 2012. But I do not see where I can define detection rules for a package. The only thing I can do is to define what o/s I can deploy to. But I would like to detect file existence before deploying the package. How can I define this rule in a package?

Any response will be greatly appreciated!!!