Validate JSON?

You don't.  Dreamweaver doesn't have built in JSON validation. 

Use this online service.

https://jsonlint.com/

Not sure if DW can validate json files as I don't use the program anymore. Have you tried manually creating a file with the .json extention and inserting a bit of json formatted data into it to see if it detects any errors?

Failing that there are free online json validation websites where you can paste your data and they will flag up any errors.

Failing that consider moving to an editor which provides json validation as default, if its that important for your workflow.

Usually you would let the back-end do the json formatting for you before it gets to the front-end. Most times when that doesn't happen its usually because there's not a lot of data so it can be manually produced. However I would NOT advise trying to manually produce json data beyond a limited amount as it soon gets unmanagable.