Is it unrealistic to expect to find an image of another coin struck with the same die pair as mine?
Not unrealistic at all to try! But it is also possible you may never find them, depending on the coin. For this particular type, I honestly don't know how hard can it be. Also, keep in mind that you can probably find only one die match and not necessarily both obverse and reverse matching at once.
Is there a way to tell for sure what the denomination is?
Usually coin measurements are enough, generally speaking of course. Flan size is by far the most notorious way to tell for well patinated coins, where asses and sestertii coexisted with similar designs.
Also, an as from this era will be made of copper whereas a contemporary sestertius should be made of an alloy identical to bronze. If by any chance some of the bare metal can be seen (As in a scratch, for instance) beneath the patina, that's another way to tell.
This coin looks copperish, but from the pictures I am not able to tell if that's just how the patina looks.