Weight. The most important thing to do is weigh the coins using a fine jeweler’s scale. We weigh every piece we buy from the public, and this is the quickest and most definitive way to spot counterfeits. It is extremely hard to make a counterfeit coin appear correct dimensionally and also have the correct weight.
Visual Inspection. As you gain experience in the industry, you start getting a feel for things that just don’t “look” right. If you are buying products you previously own, American Silver Eagles for instance, you can quickly compare your new purchases to your old ones to verify that the design and intricacies appear the same on both coins. Even small differences are certainly causes for concern.
Know Your Seller. By far, the easiest way to avoid counterfeits is to buy from reputable dealers. The odds of purchasing counterfeit material from a major dealer are extremely low, and those odds increase as you buy from individuals or private sellers on eBay or Craigslist or such.

Ring Test (video) – A silver coin or bar will have a distinct ring, as opposed to fakes which will have a thud when struck or dropped.
Weight (video) – Understand that a “Troy Ounce“, which is how we generally weigh precious metals, is different from the popular “Avoirdupois Ounce” used as a more traditional unit of measure in the United States. Just because a coin or bar says it weighs a certain amount doesn’t make it so. If you have a gram-based scale, bring it with you to the coin shop or Craigslist exchange. If you don’t have one, spend $30 and pick one up before you spend thousands on precious metals.
Nitric Acid Test (video) – You may not be able to test every coin or bar with nitric acid, as it requires a little bit of filing down to get under the “plate” but if you are buying in bulk, the seller may allow you to test a random piece of your choosing after you’ve performed a magnet, weight and ring test.
Coin Caliper – If a counterfeiter uses a metal other than silver, chances are that the coin dimensions will have to change – or the coin will weigh more or less than it is supposed to with the specific dimensions. Every minted coin has a specific diameter and thickness. A caliper, usually available for $15 – $50, will give you the ability to measure the specific inches/millimeters of a particular coin. Cross compare this information, along with the weight, to the mint’s coin specifications and if they match up, then the likelihood of a fake is extremely low – especially if it “rings true.” (Source)

Weight. The most important thing to do is weigh the coins using a fine jeweler’s scale. We weigh every piece we buy from the public, and this is the quickest and most definitive way to spot counterfeits. It is extremely hard to make a counterfeit coin appear correct dimensionally and also have the correct weight.

Andrew Greenham from Forest City Coins in London, Ontario, Canada, obtained the counterfeit from a second Toronto dealer who had acquired 10 examples from yet a third, unidentified, Toronto dealer, who was duped into purchasing the pieces from an unidentified seller, as genuine silver Eagles.

50.4753 percent nickel
39.3614 percent copper
10.1163 percent zinc
0.0271 percent gold

... avec une bonne balance autre que ces balances chinoises à petits prix mais peu fiables.

Tu es un peu sévère
Pour moins de 10€ on trouve des balances chinoises suffisamment précise pour détecter les faux !

Du zamac, une des raisons pour lesquelles je n'aime pas trop l'argent-métal : un bon zamac fait facilement illusion. Remplacer de l'or par du laiton, on voit très vite la différence.

Tu es un peu sévère

+1 : la mienne est largement assez précise pour détecter les faux.C'est aussi très rapide : pour des pièces identiques, on en passe 10 pièces à la minute, en laissant chaque pesée stabiliser. Soit 2 minutes chrono pour peser un tube d'ASE une à une.Pour le particulier c'est largement suffisant.Et si on veut vraiment la qualité, c'est non seulement une meilleure qualité intrinsèque, mais c'est aussi un importateur avec une garantie et un SAV, et là ça va pas être qq euros de plus, ça va être beaucoup d'euros de plus.