I don’t think I know of any toml validator or any netlify.toml validator - we certainly don’t provide one.
If you can format your toml badly enough, your deploys will fail, but that’s more a “it’s totally broken” than “it’s provably correct and does what I mean” kind of validation.
I do think it’s a reasonable feature request, so I’ll get that filed for us, so I can follow up here if we end up creating one.