It appears when you view the information on an app you have installed, there is a new line of detail under a heading labeled "Store" which will show if you have installed an app from the Google Play Store or if you've sideloaded it (aka the Package Installer). Unfortunately, these seem to be the only two options that will be shown though. We tested out installing apps from the Amazon App Store, but even that simply showed as coming from the Package Installer.
Of course, you may wonder what use this type of information may be. Some have claimed it may lead to some DRM issues or a crack down on sideloading apps, but there is little reason to expect DRM troubles from this. We can imagine enterprise security companies using this information to limit sideloaded apps on employee devices in order to minimize troubles from potential malware though.
source: Android Police