How to know if my iPhone is jailbroken? - 1

I have the iPhone 3G one. It used to be my dads, and I was sure he jail-broke it because he would use it to "tether" and also he had a weird "slide to unlock" button (it was a Pokemon ball instead of the arrow). He gave it to me and everything on it is gone. No traces of pictures, apps, etc. It's like new. I heard one way to find out if it's jail-broken is to look for Cydia, but there are no apps on the iPhone. Did he somehow… Un-jail-break it? Lol.

If you restore it to factory settings it's essentially un-jailbroken. That's probably what he did, being the responsible man he is

No Cydia, no jailbreak.