Although the net is plenty of sites for validation of a payload against a json schema, below you have this service.
Besides this post the form will be available in the validation payload link where it is explained how to validate through an API call (bottom part of the page, so scroll down a bit)
For those of you willing to submit new data models there has been an update of the contribution manual in section 1.2 pages 9-11.
It affects to the use of $ref for single properties and how to document.
Additionally the expections allowed (for the moment) for the documentation of geoproperties.
The API call for checking a json schema submission before being accepted into the data models are already available here:
Now it can be also used through a form like this one in this other link.
Check it out below