The "Show preview" button can be found at the bottom of the "Save dialogue" popup window. The Save dialogue is the popup window shown when clicking "Publish changes…" at the right-side of the toolbar at the top of the Visual Editor. Pressing this will show you what the article will look like without actually implementing your edits (i.e. publishing your changes online.) It is strongly recommended that you use this before hitting "Publish changes", which will cause your edits to be published onto the page and made visible immediately to everybody.
In the source editor, the "Show preview" button can be found at the bottom of the text editor box.
Discovered by embedding cosine similarity (sentence-transformers MiniLM, 384-dim).