The Marketplace Tools directory on GitHub contains standalone tools that developers can use to validate extensions before submitting them to Magento Marketplace. Look for additional tools coming soon, including a validation script for Magento 1.x and a code sniffer.
Package Validation Script
|Description||This script verifies that a package meets Composer requirements, and is ready to pass the validation step of Technical Review. If the debug option is set, a report is generated that might include a combination of errors, warnings, and notes. All errors should be fixed before submitting the package for technical review. Any warnings encountered can be fixed at your discretion, and will not prevent the extension from passing validation. To learn more, see Marketplace Tools on GitHub.|