Currently the following two expressions return false
ResourceResolverImpl.isResourceType(<Resource with resourceType="sling/some/type">, "/libs/sling/some/type"
ResourceResolverImpl.isResourceType(<Resource with resourceType="/libs/sling/some/type">, "sling/some/type"
Since it cannot always be influenced whether the given resource is absolute or relative (because both usually works from a rendering perspective when you talk about the current request's resource), both cases should actually return true.
See also the related discussion at http://firstname.lastname@example.org/msg62351.html