In general, IR is pretty useful to determine if you have a specific functional group in the molecule or not, it's not really the best for piecing together the connectivity (that's why you only need to identify a few key peaks in this case).
So for your problem...looking at benzamide what are a few key structural features/functional groups that you could use to differentiate it from another molecule? Specific functional groups will have different peaks in the IR spectrum. For example, if you were looking for a C-OH (alcohol) in IR, you would hope to see a very broad peak around 3200-3400 (cm^-1). Then just match up your functional groups with the spectrum that says they are present.