When you reach the Awazon login, the page prints the market's own onion address into or beside the captcha image, generated server-side. Your job is to hold that printed address against the one in your browser bar.
If they match, you are on a genuine address. If they do not, close the tab, because you are on a clone. A phishing site can copy the Awazon theme in minutes, but it cannot easily serve the correct address in the captcha while your bar shows a different one. The mismatch is the tell.
Do this every session, not just the first, because a saved bookmark can point at an address that has since rotated out.