Using reflection to walk the Interop for your COM DLL will give you classes and interfaces that a vaguely analogous to COM CoClasses and interfaces.
If quick and easy is what you're after, add your COM DLL as a reference in Visual Studio and use the code below to walk through the Interop assembly looking for classes. If it's a VB6 COM DLL everything is going to be IDispatch anyway, but you could optionally filter on objects with a CoClass, Guid or InterfaceType attributes on the types identified below (you didn't specify whether you were after classes or CoClasses).
var classes = Assembly.GetAssembly(typeof(MyComDLL.RandomTypeFromWithin))
.GetTypes()
.Where(type => type.IsClass);
If you're trying to really do this properly, the articles Dennis linked to in his comment are better, as is p/Invoke'ing LoadTypeLib and walking ITypeLib (the latter option being non-trivial).