In a document style SOAP message that follows the WS-I Basic Profile, the SOAP body may only contain one XML fragment.
This XML fragment can be validated in its entirety, which means that the root element of the fragment belongs to some namespace.
With RPC style, the root element will be an element which corresponds to the name of the operation invoked and parameters to the operation, which are sub-elements of the root element, will thus be inside this element.
The root element in an RPC request is not described in the WSDL and need thus not belong to the same namespace as the parameters to the RPC operation.
So, at least in theory, it should be possible to tell the difference between a Document style and a RPC style SOAP message by looking at the namespace of the root element of the payload of the SOAP message.
Hope this helps and that I got this message right, writing it in a hurry!