Dealers don't want people to think that there is much fake silver out there, as scared customers reduces demand for their products. However, we think it is important for you to understand the risks out there, and be informed -- so you will be less likely to suffer a loss due to counterfeit or fake silver bars or coins.
If you are looking to buy silver pandas, the best thing to do here is buy from a knowledgeable dealer, who sells a fair amount of Chinese pandas.
Apparently, at some point in the 1980s, it was discovered that some 100 ounce silver bars were drilled and filled with lead. They are reported to weigh the same as real bars, and to be virtually indistinguishable from real bars. However, they are not as prevelant as one might imagine. And, they contain about 25 to 45 ounces of silver, so the bars are not a total loss.
Reports of tungsten-based 400 ounce gold bars have occurred as easly as about 1982 (per Precious Metals 1982, by International Precious Metals Institute, p.657 -- "there are 400 ounces and kilo bars with tungsten cores bearing real looking but counterfeit markings of respected refiners.")
However, this report has plenty of holes. First, there are no sources. Then, the first report and second conflict (real bars that are gutten and filled with tungsten, versus tungsten blanks that are gold plated). Then, there's the fact that there are precise details (5,600 to 5,700 bars, 1.3 to 1.5 million blanks), which if true show that sources include Hong Kong officials *and* United States perpetrators, yet the alleged refiner wasn't named.
Even more telling is the fact that the United States currently only has 2 refiners approved for London Good Delivery Bars (Johnson Matthey and Metalor USA Refining Corporation). So there aren't a huge number of potential candidates. And the fact that Chinese officials apparently had 'many' of the perpetrators in custody (yet most of the perpetrators would have been in the United States). Plus, there's the fact that getting 5,600 400 ounce gold bars into the London Good Delivery system isn't easy.
Also, 520 million ounces of gold is more than half of what all the governments in the world have in reserves! It is also 6.5 years worth of global mining production. A refiner putting that much gold suddenly into the Good Delivery System would likely raise a lot of suspicion. They also claim that 640,000 bars were shipped to Fort Knox -- yet 640,000 400 ounce bars is 256 million ounces, yet the United States has 261 million ounces of gold. So they are claiming that 98% of the gold in Fort Knox is all fake, made from a United States refiner in the mid-1990s. That is very hard to believe (and if true, where did the real gold from Fort Knox go?).
This is true, however there is more to it than that. Silver is considered diamagnetic (as is lead), meaning that it repels a magnet (as the magnet moves). Depending on the amount of silver you are testing, and how strong the magnet is, you may or may not be able to detect this. If you move a strong magnet over a 100 ounce silver bar, you'll feel resistance (similar to moving your hand in water, rather than air). Also, if you drop a magnet from an inch or so above the silver bar, it won't fall quite as fast. And if you place the bar at a 45 degree angle, the magnet will move slowly down the bar (compared to a book, where it will move as fast as expected).
Real silver (or gold) will not be attracted to a magnet. If you have a magnet (preferably a strong one), and it sticks to the suspect coin/bar, then it is not real silver. Note, however, that lead (and copper and tungsten) are not attracted to magnets either, so this test won't reveal a lot of fake silver. Most likely, this would work best with fake silver jewelry.
For more way to test silver, check out SilverCoins Fake Silver Coins: 11 Ways to Spot Counterfeits.