How do I test the EPUB before publishing?

Always page through your EPUB in an actual reader before publishing:

• Apple Books (Mac/iPhone/iPad) — drag the file in.
• Calibre (free, all platforms) — a great desktop EPUB reader and library tool.
• Thorium Reader (free) — a clean, standards-focused reader.
• Kindle Previewer (free from Amazon) — the best way to see how it'll look on Kindle specifically, since Amazon converts your EPUB.

Check that chapters break correctly, italics and emphasis survived, and the front matter reads well. Try changing the reader's font size to confirm the text reflows cleanly — see Why does my EPUB look different on different devices?