When I manually want to check for a valid json, I have always used jsonlint. I discovered that jsonlint has an npm and ruby package.
You can find the plugin here: npm or ruby.
You can either install it with
npm install jsonlint -g (for npm) or
gem install jsonlint (for ruby).
You can now use it in terminal like this
jsonlint filepath.json. This will output nothing if it was successful or it will print where it went wrong.
Or if you want to use it in a script that will check for the validity, here is an example: