Hello Dj Cj,Thanks for bringing up your question here in the Apple Support Communities. It looks like you are seeking a way to tell what apps were purchased under which specific Apple ID. I am happy to offer some options to get this done for you.The first thing we want to ensure is that all your data is safe. You can make a backup in iCloud or iTunes. Use this article for help with that: How to back up your iPhone, iPad, and iPod touch.Since you are signed in to the iTunes and App Store, please click on the App Store on your iPhone 6s Plus. On the bottom right corner of the App Store that opened, there is the "Updates" section. Click that. Once in there, you can click the top area that says "Purchased". This area shows you all the items purchased under that Apple ID. There is also an option to click the "Not on This iPhone" tab, to see what apps were purchased with that Apple ID, but are not currently on this iPhone.This option will need to be done for each Apple ID you own. If you are specifically looking for apps that were not purchased or downloaded (if they are free) using your own Apple ID, you will have to compare the list you get for the apps under your own Apple ID, and look for that particular app. I do want to warn you that switching your Apple ID means you associate your iPhone with a different Apple ID. This may cause an alert such as:
"This device is associated with another Apple ID. You cannot associate this device with another Apple ID for [number] days." Depending on when you associated your device with your Apple ID, you might need to wait up to 90 days to associate the device with another Apple ID. This information can be found in this help article: View and remove your associated devices in iTunes. If you see that message or alert, you will not be able to change your Apple ID (or change it back) until that number of days has passed.
If you need assistance signing out and signing back in with your Apple ID, please check this help article: Sign in with a different Apple ID on your iPhone, iPad, or iPod touch.
The best option I can offer to check all your purchases for a specific Apple ID without changing the Apple ID associated with your iPhone is to open the App Store on a Mac computer, sign in with your Apple ID you wish to check, and click the Purchased tab. You can take a screenshot of your purchases if you wish. Then, sign out of the App Store with that first Apple ID, and sign in with any other Apple ID you own, and click the purchased tab again. That will show you all purchases with those specific Apple IDs, and not cause you to wait up to 90 days for the "associate this device" alert.
You can take a screen shot on your Mac using this help article: How to take a screenshot on your Mac, or on your iPhone using this one: How to take a screenshot on your iPhone, iPad, and iPod touch.
Thanks again, and have a great rest of your day.