How Can I Log Out Of iCloud From iPhone? Here's The Solution.
I should give you a hint on what account is used. Check the App Store and scroll all the way to the bottom, it usually shows the account. Also, check the iCloud on the device, you can find the account from there.
The new security settings in iOS 8 and up requires you to have the account and password. You can recover your Apple ID at
https://support.apple.com/en-us/HT201487.