Good day, to easily check if your iphone is factory unlocked or sim locked to a specific carrier for free, go to
iPhoneIMEI.info :: Free iPhone Unlock Checker
Sometimes the site is down but when it is up and about, it can give you a heads-up if it is ok to upgrade to the latest ios version without the risk of not being able to activate the iphone for use.
For all those planning to upgrade and jailbreak, keep in mind that the latest ios Apple servers are signing is ios 6.1.3. this is the only ios version that you can upgrade to as of now, and that version is compatible for jailbreak. So if you are on ios 6.0 to 6.1.2, stay on that version and just jailbreak.