DM is most likely the maker's mark - the name, logo or initials of the jewelry's manufacturer, importer, wholesaler or retailer.
But unlike trademarks which have been registered with the U.S. Patent Office or with another country's patent office, maker's marks are unregistered.
And because they're not registered, there could be a number of them stamping the initials D and M on their creations. So, your best way of knowing if what you have is real, have it checked by a professional.
However, there are simple ways that you can try at home to test a gold piece's authenticity (thanks to SBCGold.com):
The Acid Test
Genuine gold will stand up to your attempt to conduct a nitric acid test at home. Make a tiny mark on the piece of gold to penetrate the surface. Drop a small amount of liquid nitric acid on that scratch and wait for a chemical reaction. Fake gold will immediately turn green where the acid is. Gold-over-sterling silver will become milky in appearance. Gold will not react to the nitric acid.
The Magnet Test
Genuine gold will not attract a magnet. Imitation gold and other alloys will attract the magnet. Coins can appear to be gold, when in fact the surface is simply gold in appearance.
Using the Float Test
A cup of water is all you need for another important test. Any size piece of genuine gold will immediately sink to the bottom of any liquid. Imitation gold floats or hovers above the bottom of the container. In addition, real gold will not rust or discolor when wet.
Skin Test
Skin discoloration from wearing fake gold jewelry is not a myth. For this test, simply hold the gold in your hand for a few minutes. Perspiration on the skin creates a chemical reaction with the gold. Skin will discolor (a black or green color) if the gold isn’t real. Genuine gold will not react with the skin.
Tip! Learn How to Care for Gold Jewelry at Real Simple.