How to find a UTM zone of a layer?

I want to find the UTM zone of a layer. The scenario is that I created a new map document which doesn't have any coordinate system defined. Then I added a layer (provided by instructor) in the document. The layer is Aerial_Photo (an aerial photograph) which has unknown coordinate system BUT the EXTENT in the source tab tells me that it has six digit right and left extent and seven digit top and bottom extent. This means it has UTM coordinates. My instructor has told me that the system is UTM WGS 84 system. But the question is which zone of UTM is that layer in? How can I find that? Please help