Get this: http://www.dependencywalker.com/ , use depends.exe
to open the DLL, then activate "Undecorate C++ Functions" in the "View" menu. I mainly use it to find dependencies, but it also exposes entry points of DLLs.
This is not fool proof, because a DLL that exposes a class doesn't have to export its methods. For example, pure virtual method layout is sufficiently uniform that you can expose your instances as interface pointers with maybe a factory function. But it might solve your problem.
And regardless, you need a copy of dependency walker. :)