I bought a chinese phone thats seems to be be already rooted. How can I be sure? When I download apps that require root they don't work. Can I change privledges on an already rooted phone? Or can I re-root?
You can try installing the app Root Checker to know if your device is rooted or not. And yes you can root your device again even if its rooted. But yes first its better to check if it has the rooted privileges or not. As far as running of apps is concerned sometimes some app stop responding even if one has the rooted privileges better would be to first check things out.
In the literal sense, no—you can't root a phone that's already rooted, because that's like killing a bug that's already dead.
If the real question is, as I understand it, whether it's okay to go through the rooting process on a device that's already rooted, generally yes, there's no reason why that would cause any harm. So if you don't know whether the device is rooted, and you want to go through rooting process just to make sure, there's no reason to be concerned that you'll be causing harm if it happens to have been previously rooted. To extend the analogy, it's like not knowing whether the bug is dead and spraying it with RAID just to be sure.
However, it's a waste of time to root a device that's already rooted, because the rooting process is always more complicated than checking whether you have root access. You can use Root Checker as suggested in Peter Carlos's answer. Rather than installing an extra app just to check for root, though, the way I do it is by launching Terminal Emulator (an app you'll probably want anyway on a rooted phone) and issuing the
su command. If you don't have root, you'll get an error message like
/system/bin/sh: su: not found
If you don't get any errors, you have root. Also, the prompt should change to begin with