Validation for RDF can mean a variety of different terms especially where RDF is using XML and several layers of technology are connected. This FAQ describes validation for RDF and answers how to do it for the different technologies.
Validation is a tricky word to consider, and often used with schema, which can also have several different interpretations. There is validation of syntax (XML validation, RDF/XML - RDF's XML syntax) as well as RDF schema validation.
That means you can do:
RDF schema validation bears expanding. An RDF schema is a description of the terms used in the RDF triples forming the RDF graph (which can be written in a document, in an RDF/XML syntax). Checking the terms match how the RDF schema describes them is what RDF schema validation typically means. RDF schemas allow description of classes, properties and the ranges, domains of properties and so on. This is explained further in the RDF Vocabulary Description Language 1.0: RDF Schema W3C Recommendation.
There are plenty of tools that do these kinds of things already. The XML validation needs an XML parser, a description of the XML - an XML schema in some XML schema languge and an XML schema validator. The RDF/XML validation needs an RDF parser (see the FAQ How Do I Parse RDF?) The RDF schema validation needs a system that do the checking which formally, is called handling RDFS entailment defined in the RDF Semantics W3C Recommendation.
You can find RDF schema validators in free software RDF toolkits such as Jena and Sesame, Euler (all in Java) as well as in the python Cwm. There are some on-line RDF schema validators such as Rosco - a non-judgemental RDF schema and document checker which does checking, but not all RDFS entailments.
OWL systems which contain OWL reasoners can also typically validate RDF schema since it is a small subset of the more powerful things that OWL reasoners can do, and all RDF is OWL Full. OWL applications that only handle OWL-DL or OWL-Lite cannot check all RDFS entailments.
The W3C RDF validator is an RDF/XML validator, not an RDFS validatior. It is based on the ARP2 RDF parser which is part of Jena. There are many more RDF parsers available that can perform this as already described in the FAQ How Do I Parse RDF?
Finally, you can do XML validation of RDF/XML against an XML schema such as RELAX NG. It's in the RDF/XML specification in section A.1 RELAX NG Compact Schema. The W3C XML Schema language (WXS) is not as suitable as RELAX NG for XML-validating RDF/XML, since it is enforces strong XML constraints and RDF/XML has a wide-open set of tags that may appear.
See also the FAQs: