Go in for a fitting, but part of what they are going to check is:
Like the middle part need to be against your breast bone. If you have enough room to shove lipstick, your keys, cell phone, your wallet and who know what else in there the cups are too small. The goal is to lift and separate. Also the side of the wire needs to be around where your arm is, but not under your arm, that it pokes.
Another sign is the dreaded quadraboob. which is where the b*** over flows the top of the cup and the cup digs in enough to make a line, and it looks like you have 4 boobs not 2.
The trick is to find the cup that fits first then adjust fit with the size of band. Of course, like Kristy said, sometimes you are limited by what the store carries. Underwires, are the easiest to fit, but not always what people want, though in most cases the right fit in an underwire will be comfortable.
cup sizes have a relationship with the band, that is a little confusing unless you see it on a chart. but it goes like this:
32f = 34e = 36D= 38c= 40B= 42A All those sizes have the exact same cup, the difference is the ratio of how much the b*** sticks out from the torso, extra weight plays a role in this. 42A stick out about an inch, 36d stick out about 4. The 36d girl looks like she has bigger boobs, but as far as breast tissue alone, it is the same, its about proportion.
so if you need a 36e (DD) and the store does not have one, try a 38D, the cup will be the next size bigger. the band might be too big, but it helps in fitting to find the right size cup first, (and you might have to resort to mail order or specialty stores) once you find the right size cup you can adjust the rest.
If you do have to resort to mail order try to stay within a brand, sizes tend to run truer in brand, especially the more pricier ones, but damn its sucks when your bras are $32 or more a pop. Though I have gotten one that was obviously labeled wrong, so it is harder. And styles within a brand may vary, that you have to shift along the line of the same size cups. if there is any padding (or mimizing) going on that will change things a little too.
My grand mother had humongous boobs, she wore a 44dd, I suspect she needed a 38H (or an even higher letter) but just made due. Especially in her 80s, she was not that "big" of a woman overall. But meh, she reasoned they were cheep at wal-mart an no one saw them anymore anyway. Her boobs were so not pretty in the end, though they probably would have sagged anyway, but she never wore the right size bra.