How to know if my iphone is jailbroken?

I want to switch my number onto an at&t iphone. I installed cydia and didn't do anything else. Would I be able to switch numbers now? How do you tell if the phone is jailbroken?

Added (1). I have sprint currently and that's why I needed to jailbreak it to switch numbers

If the phone has Cydia on it, it's jailbroken. Another way to tell is if when you power it on (not wake it up), the startup screen is something other than the normal Apple logo.

But this shouldn't matter for switching numbers; you should be able to do that whether or not it's jailbroken.