The key is that the server is free to tell the client that there does exist structure in a given URI-space, and then the client is free to make use of that knowledge in future requests. That is how server-side imagemaps worked -- HTML says that the URI is structured such that appending "?X,Y" to that URI, where X and Y are non-negative integers, corresponds to points on a map that can respond to future GET requests.
Thus, one way for the server to tell the client that a given URI is structured is to provide the URI in a standard element of a standard media type that has been defined as such. Another is to include the URI in a response header field.
I may be having my own, belated moment of total, violent, zen clarity.