Sango Productions's answer provides sufficient RAW information. This answer is a bit more frivolous, and goes over the in-fiction reasoning our table used when this came up.
We allowed bat-like wings to be concealed beneath cloaks (or even to be used as cloaks) on the basis that it looked pretty cool on Gargoyles:
In real life, bats have very flexible wings (but not as flexible as the gargoyle above). With dragon wings being modeled on bat wings, this seems like a reasonable example to use.
This little guy could pretty easily hide his wings under his cloak:
Although with his wrists that high it would probably be clear that he was hiding something (hunched back, armored shoulder-pads, back mounted weaponry, etc.), or that he had a very thick cloak.
For bird-like wings, it's worth noting that while the typical representation of bird wings are quite large while folded:
Actual birds can usually fold their wings quite close to their bodies, with their wrists not much higher than shoulder level:
Of course, this all pre-supposes that the DM is okay with it and just looking for a reasonable explanation. There are plenty of reasons that this may not hold true in any given campaign:
The DM feels that dragon wings are more muscular / less flexible than bat / bird wings.
The DM feels that the square-cube law should be in effect, resulting in crazy-huge wings for humanoids.
Since there does appear to be a bit of a vocabulary mixup for this word, here's what I mean by a cloak:
It is a subset of capes, designed to wrap entirely around the body when necessary.
It is distinct from a robe, in that it is open in the front and worn over other clothing.
It is distinct from a coat, in that it generally doesn't have sleeves.