Yes. Real 925 silver darkens over time as it reacts with sulfur in the air, then polishes back to bright. Tarnishing is normal and is not a sign of a fake.

No. A 925 stamp is a strong clue, but counterfeit pieces are sometimes stamped too, and genuine older or handmade silver is sometimes unstamped. Only a test confirms it.

No. A magnet only rules silver out. If the body sticks it is not solid silver, but many base metals are also non-magnetic, so no pull does not prove silver on its own.

Yes. It comes down to the base metal under the gold, not the plating name. If that core is sterling silver the piece is real silver, and a 925 stamp is the tell.

A jeweler's XRF scan is non-destructive, fast, and usually free. It is the practical way to be certain without damaging the piece.