does my dll support 64 bit?

The simplest way is to use a dependency walker, like this one: http://www.dependencywalker.com/[^]. It should show you which libraries are not really compatible with your application.
But it can resolve only library-level dependency constraints. And there are many situations, when the referenced dll is ok, but the api behind the function it calls works differently in x68 and x64 environment. It is not the case in pure managed libraries and framework libraries manage it also, but you need to be careful in case of third party libraries doing API calls (p/invoke). So I suggest you use reflector on such libraries, and check every referenced function. If it references a winapi, you can check on msdn, if it is a call to unmanaged library, it can be difficult...