
To see a document's fonts in Sketch, you need to either have the font installed on your Mac or embedded in the document itself. If neither of these is true, you'll see a missing font notification in the Mac app's toolbar or in the web app when you open an artboard.

The Fonts tab in the Mac app’s Document Settings panel lists missing fonts:

Keep in mind that missing font scenarios can occur in different situations:
If you don't have a font and can't get the files, ask the person who created the document to embed it. If a font is already embedded in a document, you can access it by going to the Document Settings panel. To embed a font yourself:

Replacing Missing Library Fonts. To replace missing fonts in a Library, you must have edit permissions for the Library file itself.
If you can't replace the fonts in a Library (for example, if you don't have permission to edit the file), you can disconnect the Library from your document. This is useful if the Library contains symbols you no longer need. Once disconnected, the symbols from that Library will become local to your document, allowing you to replace the fonts there.
You can use a plugin like Library Unlinker to quickly disconnect the Library from your document.
If you've already checked your Library Layers and Symbols and still see a missing font notification, the issue is likely in your text styles. At least one text style is probably using a font that is either missing or needs to be embedded. Once you find the font, you can embed it or replace it. Be sure to save and close the document for the changes to take effect.