Testing first. There are many ways to test for gold purity, ranging from mass spectrography (you need a university in your pocket for that), refining (you need a furnace), various electronic testers (too expensive unless you’re a professional) or you can by a few acid solutions and a touch stone from your local jewellers supply. This is what I have used over the years and it has seldom let me down. (you cannot buy these online as they are very powerful acids and shipping these by mail is banned AND don’t get any on you!)
TOUCH STONE TESTING. The acid solutions are marked with the karat gold you are testing. You must have samples of known gold to compare with if you use a touch stone. (A touch stone is a ground black glass sheet ot the like). You take your unknown piece and rub it on the touch stone leaving a mark. You then rub known karat golds next to it and then apply the acids. The unknown gold (generally the mark disappears) will react the same as one of the known golds. And thus the unknown becomes known.
I personally use the next method as it is more accurate.
ACID TESTING THE ITEM DIRECTLY. Because you are only rubbing surface gold onto a touch stone, you are not testing what’s really inside. It may be just thick plate. What you need to do is go through the surface of the gold by using a file or blade in an INCONSPICUOUS place. A good place in the case of a chain or bracelet is to do it inside the links or clasp where there is already a lot of wear. File into it a small amount and then place a drop of, say 9K acid, directly on the cut. If no reaction then it is at least 9K. Use the next highest acid (say 14K). No reaction then try 18K acid. No reaction? Then you know it’s a least 18K gold. The reactions you normally see are as follows. If the cut goes brown then it’s gold but less than you acid strength. Try a lesser acid until no reaction. That’s your karat. If it goes green and may bubble, it’s got copper in it and is plated gold. If the acid goes milky white and the metal black, it is silver. The only reaction you want to see is a brown tarnish on the gold….All else is NOT solid gold. Remember you want to test well below the surface.
FAKE GOLD. Things to look for that should raise your suspicions.
Look for discolouration. Plating wears and the metal beneath may show. Inspect between the links and near the clasp where the piece rubs the most. If in doubt, test.
Anything lacking known hallmarks should be questioned. There are a number of reasons that the marks may not appear. The piece may have been repaired. A ring may have been resized. A piece may be worn enough to rub away the hallmark. If in doubt get an unconditional guarantee and then test.
Look at the links. ALL gold jewellery has EVERY link soldered closed. Gold is a precious metal and NO self-respecting jeweller would leave links open so you could lose the piece easily. If any link is unsoldered, test.
Most fake gold pieces are heavy. I have yet to see a lightweight counterfeit. That’s because it’s only worth forging heavy pieces. No point in breaking the law for peanuts.
Many other tests only come from experience. A few are a bit strange but are enough to raise your suspicions. Gold is a very heavy metal and over the years you learn the “feel” of it. I can generally tell if it’s ok by the weight in my hand. Gold is a soft metal. With high karat gold (22K,24K) you can leave a mark with your teeth. I’m sure you’ve all seen it on TV. It works but don’t try it on 18K or less if you like your teeth.
Many have suggeted a magnet is a good test. I have yet to encounter gold plated steel jewellery so I don't think that test has much use. However it is simple so you might as well.
An interesting one is the smell test. When people say they can smell gold they may not be far wrong. If your hands are sweaty, rub the gold jewellery vigorously in the palms of your hand. If it’s plated you may smell a strong acidic metal smell. That’s the electrolytic reaction of your salty sweat on the plated metal. None of the last few are to be relied on…. but at a pinch?….
So the best thing is to buy a gold testing acid kit. It will have full instructions. Next, make sure you get a guarantee as to the gold you are buying. Check it yourself if you feel confident or have it checked by a professional.
In 25 years of dealing in secondhand jewellery, I have only had a few pieces that were counterfeit. It’s not that common. The internet may have changed that.
Hope that helps.
Please have a look at my other guides on gold jewellery to expand you knowledge, and remember, this is not professional advice, it’s a guide only. If you want professional advice, you’ll have to go to a qualified expert.