JSON Validation
When working with JSON, you should make sure it's valid. You can test it out easily by pasting your JSON into the form on http://jsonlint.com/ which will format your JSON and tell you if it's valid or not.
GeoJSON Validation
GeoJSON is a special type of JSON that allows you to display geo data in a standard way. Some databases store geometries using GeoJSON as well. GeoJSON cannot technically be validated but there are a couple tools that will help you determine if your GeoJSON is good or not.
http://geojsonlint.com/ is a good resource for testing your GeoJSON and alternatively, you can paste your GeoJSON in this OpenLayers example, http://openlayers.org/dev/examples/vector-formats.html