The signed list and the header check.
Two checks confirm a genuine Mars address, and the deep pool means you always have somewhere to rotate.
The page header reprints the onion you should be on, so you can compare it to your bar. A clone struggles to serve the correct address in the header while pointing you at the wrong one.
Mars signs its deep mirror list with a PGP key. Verifying the signature proves an address came from the market. When one address stalls, do not wait, open the signed list and move to the next live onion.
Mars Market