As far as I know the return type is not stored inside the DLL at all, it's not necessary for the way DLLs work and it's the responsibility of the caller to know what it expects..
The same for the parameters. You get no clue as to what they're for unless you disassemble it and try to understand the assembly. One thing you can find out though is the name of the functions and
how many parameters they take.
To do this I suggest you download
OllyDbg and open your DLL with it. It'll say that it can't execute them directly and ask if it should launch LOADDLL.EXE, say yes.
Once the DLL is loaded, click on the triangle "play" button (or press F9) so that it executes the DllMain() function. Then go to the Debug -> Call DLL Export menu and you will be provided with a list of functions that were exported in the DLL and (if it is able to find it out successfully) the number of parameters they take.
