How to know which Apple ID was used to set up my iPhone 4s?

Well I currently live in Peru but I got a brother living in the US who sent me his old iPhone 4s to Peru. It's working fine by now (currently using it as an iPod) but it's got iOS 6 running and I wish to update it to iOS 7. However, I was told that after installing I'm gonna be asked to put the Apple ID that was used to set up the iPhone (original owner) otherwise the iPhone would be locked. So I asked my brother for the mail and password and he gave me a working Apple ID with 3 paid music albums and some other stuff but he told me that he wasn't sure if that was the ID he used to set up the iPhone since he had 2 accounts (he doesn't remember the other one). Is there a way to prove if the ID is correct or not BEFORE updating to iOS 7 so in case it's wrong nothing really happens?

Call apple

I thought you can use another apple ID to update your phone or do all the stuff.

1. Prove whether the ID is correct is easy, just go app store and choose one of your apps to updata, then it will ask for the Password, while the ID which purchased the app would display.
2. If you forgot the ID, you can go to iforgot to get the id back by answer some questions.