How to verify you are on the market
The block at the top holds the nexus market onion addresses this site publishes. Verification has two parts: the address before you connect, and the address after.
Before you connect
Compare the address you are about to open with the three above, character by character. Not "it looks the same". Same length, same letters, same order. A v3 .onion address is 56 characters followed by .onion. If the one you have is 55 or 57 characters, stop. You do not have one of these.
The copy buttons exist because retyping is where it goes wrong. One wrong character is not a typo of the market. It is a completely different hidden service. Possibly one nobody runs, possibly one someone is running to watch you.
After you connect
Look at the address bar of the loaded page. Take the last six characters and compare them with the last six of the address you opened. This is the check that matters, and it takes five seconds.
Why the last six and not the whole string? When you copy, you already have the whole string in front of you. The check after loading is a second reading of the same data, and people stop reading long strings early. Six characters is short enough to actually compare, every time.
What a clone gets right
A fake market page is easy to make. It can have the same layout, the same logo, the same login form. It cannot fake the address in your browser. The page can be perfect and still be wrong, so judge the address, not the page.
What this site cannot prove
This page is on the clearnet. It has no view inside your Tor Browser and no way to see which .onion you loaded. If you want a guarantee, the guarantee is the check above, done by you. Anyone who offers you a stronger one from a clearnet website is describing their own traffic, not yours.