Preflight is a pluggable framework for content quality assurance. Out of the box, Preflight tests general readability (including Flesch reading level; sentence length; and word length and complexity), checks links, and provides a text replacement mechanism.
Preflight is available as an Umbraco 8 content app only, and supports content in RTE editors, textareas and textstrings, including those nested in Grid and Nested Content editors
Refer to the repo on GitHub for documentation around building custom tests.