Now that iTunes 12.7 no longer manages iOS apps, you're left with a "Mobile Applications" folder containing the apps iTunes has been backing up for all these years. For most users this folder can be Trashed after updating to iTunes 12.7. But if your Mobile Appplications folder contains apps purchased with multiple Apple IDs, you may want to re-install the appropriate apps on the appropriate devices.
But which apps belong to which user?
Apps By Apple ID will list the apps in the "Mobile Applications" folder by app name, file name, purchaser's Apple ID, size, version and last date modified.
Additionally, a tab-delimited text file of this information can be exported or the apps copied and sorted into "Purchased by..." folders (iTunes 12.7 and later only, see the Read Me for other important information).