Test yourself: http://skeeto.github.io/am-i-shadowbanned/
When a user is shadowbanned they can continue to make posts and leave comments without any indication of the ban. However, everything they do is automatically marked as spam. No one will see any of it until the post or comment is graciously freed by a moderator, if ever. To the moderator, the post title appears will appear as strikethrough in addition to the normal red background.
This tool can test whether an account is shadowbanned, normal, or non-existent. It uses CORS Anywhere to work around the cross-domain issue.