Can I verify if data protection was enabled from a backup?
You'll not likely be able to establish security after the fact directly. The protection is a combination of the hardware in the iOS device and the setting of a passcode lock and not a preference stored on a filesystem - it's baked into the OS. The backed up data is not encrypted at rest in the same manner as iOS's security. The act of restoring the backup also overrides whatever passcode lock and settings you had, so that's not a good way to proceed.

What you can do is research the exact model of the phone and the version of iOS. Grab this pdf from Apple: http://www.apple.com/business/docs/iOS_Security_Guide.pdf

Starting on Page 9 it describes the conditions needed for data to be encrypted at rest. It also has valuable information about what types of data is readable if someone guesses your passcode.

What you can do is restore your backup to the same class of hardware and observe that with a passcode set, the data protection is enabled. Hopefully you have an A7 or later processor since the PIN entry is governed by hardware rate limiters to reduce the chance your PIN can be brute forced.