A Simple Magnet
You can use a simple magnet to rule out many Chinese-made counterfeit coins because about 70% of all fake coins from China are produced with iron-based planchets (coin blanks). Since there has only ever been one single circulating U.S. coin that should be attracted to a magnet (the 1943 steel penny), virtually any U.S. coin that sticks to a magnet is counterfeit.
How to use: You will need a strong magnet to detect small amounts of iron. Hold the magnet near the coin and see if they attract, even a little bit. If the magnet sticks to a U.S. coin, the coin is virtually certain to be a counterfeit. (Note that Canada has been using steel in their coin blanks for decades, so this test has no meaning for many Canadian coins.)
A Gram Scale Accurate to at Least One Tenth Gram
Many Chinese Counterfeiters use scrap metal to make their coin blanks. As a result, the coins are usually underweight. The coins can also be underweight for other reasons, such as shrinkage from using cast dies or cast blanks. The U.S. Mint adheres to very narrow tolerances for error in weight and diameter, so any U.S. coin that is off by more than 1% is highly suspect.
How to use: Make sure your scale can weigh to the tenth of a gram (0.10) or better. Do not use diet scales that weigh whole grams because they are not accurate enough. Place the coin on the scale and then compare the weight to the known proper weight standard (see below) for that coin. If it is off by more than 1%, the coin is a suspected counterfeit.
A Caliper Accurate to the Hundredth of an Inch
A caliper is a device that is used for measuring the diameter of a coin. The U.S. Mint was extremely precise in making coins of the proper diameter, so any coin that is too small, even by a little bit, is highly suspect. Counterfeit coins are frequently underweight and undersized.
How to use: Slide the caliper jaws to the closed position and calibrate it to zero (usually by pressing a "set" or "zero" button.) Then slowly slide the jaws open until they are touching the edges of the coin across from each other. Make sure the coin is snug between the jaws (but not too tight) and read the result. Compare your measurement to known U.S. Mint standards for that coin. If the coin is too small, it is a suspected fake.
A High-Powered Magnifier or Loupe
A magnifier that has a minimum power rating of 8x (8 times magnification, or "8 power") will allow you to see details on the surface of the coin that are invisible to the naked eye. Ideally, you should use a 10x or greater triplet loupe, which has much greater clarity (but is also more expensive.)
How to use: Hold the coin in one hand, and the magnifier in the other. Move the coin around to inspect its surface, looking for signs of bubbles or pimples on the surface, or seams or file marks on the edge. Also look for characteristics normal for that type of coin by comparing to a known genuine specimen. "Soapy" looking or bumpy surfaces can be a sign of a counterfeit.
A Reference of Standards for U.S. Coin Types
In order to determine if your weight and diameter measurements are within U.S. Mint tolerances, you will need to compare them to known U.S. coin specifications. Most general-purpose coin price guides, such as the Red Book have these specifications noted. Although the allowable amount of deviation from the Mint's specifications varies for each type of coin, all of the tolerances are very, very small. Any coin that is a gram underweight or eighth inch too small is way out of tolerance and probably a fake.
How to use: Weigh and measure you coin, and compare your findings to known specifications. If they are off by more than 1% (either too heavy or too light) your coin is a suspected counterfeit.