How to identify labels in unsupervised learning?
You can't, in a completely unsupervised setting.  You need something more: e.g., a training data set where you have labels for at least some of the instances, or some other information that allows you to set a label for each cluster. 

How is the computer supposed to know that a circle means zero as opposed to some other number, if you don't tell it that?  Obviously, it can't.  In fact, in the Eastern Arabic numbering system, there's a zero-looking character that actually means the number 5, not the number 0.  Obviously, a computer can't possibly know which of those two meanings is intended, if you don't tell it somehow.