If you don't have the source code and API documentation, the machine code is all there is, you need to disassemble the dll library using something like IDA Pro , another option is use the trial version of PE Explorer.
PE Explorer provides a Disassembler. There is only one way to figure out the parameters: run the disassembler and read the disassembly output. Unfortunately, this task of reverse engineering the interface cannot be automated.
PE Explorer comes bundled with descriptions for 39 various libraries, including the core Windows® operating system libraries (eg. KERNEL32, GDI32, USER32, SHELL32, WSOCK32), key graphics libraries (DDRAW, OPENGL32) and more.
(source: heaventools.com)