Your best bet is using Ida, there is a freeware version @http://www.hex-rays.com/idapro/idadownfreeware.htm. Ida will tell you at least the calling convention and the number of parameters. If you have a PDB (eg if it's a microsoft
dll, IDA will download the PDB from Microsoft for you) for the file chances that you might get some of the parameters are likely.
What
DLL is it?