There is no public official X endpoint that returns a clean shadowban verdict for an account. In practice, people use the term as shorthand for a drop in public visibility.
That can show up as weak search exposure, thin public timelines, missing replies, or content that appears much harder to discover than expected.
Why the term is messy
Different visibility issues look similar from the outside. A protected account, low activity, deleted posts, search quirks or temporary transport failures can all be mistaken for moderation.
What a checker can do
A good checker compares profile existence, public timeline evidence, search-style reachability and historical profile stats. It cannot guarantee intent from the platform, but it can tell you whether the public evidence looks normal or thin.