How to distinguish gold from gold plated jewelry
The lack of a hallmark (10k, 14k, 18k etc.) is not indicative of whether your piece is gold or not. In the United States it is against the FTC's regulations for the jeweler to hallmark a piece of jewelry unless it is accompanied by a maker's mark. So, when I first began making jewelry and did not have a trademark of my own, I did not hallmark my jewelry. Now that I have a maker's mark, I hallmark my jewelry with the appropriate metal hallmark and my maker's mark.
A jeweler cannot just look at a piece and tell you for certain whether it is gold or not; the jeweler will do a test to determine it.
To be scientfically accurate a sample of the metal in question must be assayed in a testing laboratory, but the following two tests have been used for many years and often are sufficiently accurate for a craftsperson or the owner of the metal in question.
To answer "Is It Gold?":
With a small file, make a scratch in an inconspicuous spot. While wearing rubber gloves, use a wooden, glass or plastic stick to apply a drop of nitric acid to the filed spot. Observe the reaction. When done, rinse everything well in running water.
If there is no reaction, it's gold.
If there's a bright green reaction, it's base metal.
If there's green in the scratch, it's a gold layer over base metal (goldplate, or--if the layer of gold is thick enough--goldfilled).
If there's a milky reaction in the scratch, it's a gold layer over silver.