Most likely the structure is not too weird, like cleaved along the (5,2,16) plane. So by checking a few likely options, like 111, 100, 110, and perhaps something more, you may be able to figure it out just from the shape and size of the cell. But the more interesting question is why you don't know
What is the source of the file? If you built it yourself, a good practice is to name the file/structure in a way that it reflects the cleave plane, or make notes in a separate "lab notebook", as one would for experiments. Also useful for computations!