FHIR validation means verifying that a resource is both well-formed and conforms to the FHIR specification. This includes checking the structure of the resource, the cardinality and value domains of properties and conformance to other constraints such as profiles and business rules.
The FHIR standard defines the $validate operation for validating the instances of FHIR resources. It accepts a FHIR resource as input and returns an OperationOutcome resource that acts as a validation report.
I've created this tool to allow you to easily validate FHIR resources online. Behind the scenes, the tool invokes the $validate operation against the specified FHIR endpoint.