What do you mean validate the incoming rest message? You mean the chunk of JSON that is sent to the API? I guess you could grab the same JSON Schema and use that to validate against the json being sent in?
RAML/RAML Tools would not provide some run time way to handle this, no. RAML defines the API and any tooling for that is around helping define the API using RAML, RAML Types, etc. A generator that takes RAML (and types/schemas) to generate code would be a better bet of having some sort of runtime tools that you could use, but again, I havent seen that either.
What sort of validation are you looking to do? Just make sure it is syntactically correct/formed? Or you looked to ensure default values are set, required fields are not empty, etc? Perhaps also validating the actual values (e.g. length of strings, valid range for numbers, no weird characters, url safe, etc)?