Look for karat or fineness stamps, but remember they can be faked. A stamp is only a starting point.
A gold color cannot tell you what is beneath the surface. Plated jewelry can look similar to solid gold.
Strong magnet attraction suggests the tested part is not solid gold. No attraction does not prove gold.
Skin marks and discoloration are unreliable clues. Skip biting, ceramic scratches and home chemical tests.
Ask a professional jeweler for certainty. A jeweler or assayer can explain suitable tests and their limits.
Keep pieces separate while you arrange an assessment. Read the full guide to gold stamps and testing.