sadly, this is pretty much rote memorization (unless someone out there has some clever tactics). hopefully you don't have to memorize stuff in the fingerprint region--that would be torture! anyway, just keep doing problems, and it will become easier. also, when you're looking at subtle differences, you can rely upon the concepts underlying ir. for example, conjugation of carbonyls lowers the frequency of the c=o stretch...but c=o's in esters absorb higher than normal (~1745). also, remember that a heavier atom causes a lower-frequency stretch (think about a spring - if you placed a heavier weight on the end, it has to "drag" back slower) and that a stronger bond has a higher frequency (again, think of a stiff spring that vibrates quickly). so if you're comparing a c-o to a c=o, you can definitely say that the c=o is higher energy, stronger, and is going to absorb more to the right of the spectrum (~1700). same w/c-triplebond-c > c=c > c-c.
also, once you get to doing more problems, you'll begin to recognize the characteristic shapes of the stretches--i.e. a sharp double peak for an amine, a broad stretch for the -oh, a sharp intense peak for the carbonyl, that funny squiggly looking pattern for an sp3 alkane stretch, etc.
good luck!
