Which iPhone app version do I see on my iPad?

I have an iPhone app which has been coded to be optimised for the various iPhone screen dimensions and resolutions. I have not included an iPad version.

When I view this app on an iPad Air 2, which version will I be seeing? The version optimised for an iPhone 6 Plus? Or something else?

(My reason for asking this question is that I don't have an iPhone 6 or 6 Plus to test on so I'm wondering if my iPad can do the job for me. I'm mainly interested in seeing the layout and image resolutions).