This is a web service that returns the ActivityPub data for any URL that returns an ActivityPub message. For instance this post (https://lemmy.ml/post/19589249) returns:

{
  "@context": [
    "https://join-lemmy.org/context.json",
    "https://www.w3.org/ns/activitystreams"
  ],
  "type": "Page",
  "id": "https://lemmy.ml/post/19589249",
  "attributedTo": "https://lemmy.ml/u/hongminhee",
  "to": [
    "https://lemmy.world/c/fediverse",
    "https://www.w3.org/ns/activitystreams#Public"
  ],
  "name": "BrowserPub: A browser for debugging ActivityPub and the ⁂fediverse",
  "cc": [],
  "mediaType": "text/html",
  "attachment": [
    {
      "href": "https://podcastindex.social/@js/113011966366461060",
      "mediaType": "text/html; charset=utf-8",
      "type": "Link"
    }
  ],
  "sensitive": false,
  "published": "2024-08-26T11:43:09.033551Z",
  "language": {
    "identifier": "en",
    "name": "English"
  },
  "audience": "https://lemmy.world/c/fediverse",
  "tag": [
    {
      "href": "https://lemmy.ml/post/19589249",
      "name": "#fediverse",
      "type": "Hashtag"
    }
  ]
}

Prepend https://browser.pub/ to the URL you want to check: https://browser.pub/https://lemmy.ml/post/19589249