When parsing a RAML document, it seems that the type of a schema element is undetermined. Neither the elements of the root level schemas element or the schema associated with a body response is typed. In the case of body elements one could make an assumption about that the schema type based on media type of the response, but this seems undesirable.
One could scan the schema trying to match it against a known type (JSON schema, XML schema), but again, this seem like guessing. It seems to me that the schema elements should have an associated media type (e.g. application/schema+json). That way you’d be certain about which validator to invoke then validating the referenced schema.