Before you can worry about whether your schema is semantically correct, it needs to be syntactically valid JSON. JSON-LD is unforgiving of formatting errors: a missing comma, an unclosed bracket, or an extra quotation mark will cause the entire block to fail silently, with no error surfacing in your browser and no indication in the page source that anything is wrong. JSON-LD Validator (based on JSONLint) is the tool that catches those errors before they become invisible problems in production.
The interface is as simple as it gets. You paste in your JSON-LD, click validate, and the tool either confirms the syntax is valid or highlights the exact line and character position of the error. When you’re generating schema by hand or copying output from a generator and then editing it manually, a syntax error is very easy to introduce accidentally. I keep a JSONLint tab open whenever I’m writing or editing JSON-LD, because catching a missing bracket immediately is far preferable to wondering why a validator is throwing unexpected errors.
It’s worth being clear about what this tool does and doesn’t do. It validates JSON syntax, not schema.org compliance. Your JSON could be perfectly valid JSON-LD and still use the wrong property names, incorrect value types, or miss required fields. JSONLint won’t catch any of that. Think of it as the first step in a two-step process: confirm the syntax is correct here, then run the markup through the Schema Markup Validator or the Rich Results Test to check the semantic validity. As a first-line debugging tool it’s indispensable, and the fact that it’s free makes it an easy permanent addition to your workflow.