In today's world counterfeits represent a good business. However, it is one of the illegal businesses genuine manufacturers suffer from. This includes the field of numismatics. Counterfeit coins do harm professional coin collectors, which is why it is important to get acknowledged with the world of counterfeit coins.

Today coins are copied with intent of selling, which is illegal business, and with the gola of providing collectors the opportunity to obtain a legal copy of a very rare coin. 

Coins that are illegaly copied are made of cheap metal, such as bronze or nickel for instance. Thos coins that are legally provided for sale to collectors include an alloy of more or less expensive metals, but does not possess the same rarity as a genuine coin.

Fake coins are most ofter created after popular rarities, like gold coins de design of which was developed by Augustus Saint-Gaudens. To spot a copy at a glance is sometimes quite hard.

Illegal copies of coins are quite easy to identify, especially if paying atenting to their weight. Kafes are lighter that original coins, because they are made of cheaper metal alloys.

Original coins are very often made of such metals as gold and platinum, which are both heavy and expensive. Some hard metals, from which coins are created, cannot be easilt handled and people practicing illegal business do not have the possibillity to provide proper engraving on coins.

This Bullion Coin Collectors Site offers information regarding the already known fake bullion coins and sovereigns.

Known couterfeit bullion coins

Silver Pandas. Around 2005, fake pandas started being distributed, apparently from China, apparently made out of silver-plated copper.

2002 1/10 oz platinum eagle. This fake coin was made of a copper-colored alloy, with a white platinum colored coating, and is lighter than a genuine coin (2.13g versus 3.112g). It is believed this counterfeit was created using a computer-aided engraving machine. Someone very familiar with platinum eagles might be able to easily spot this fake.

Canadian Maples. There are likely many counterfeit Canadian maples, probably all fakes (wrong metal), but most likely all the 1oz size.

Krugerrands. There are likely many counterfeit Krugerrands, probably all fakes (wrong metal), but most likely all the 1oz size.

1994 Australian Nuggets/Koalas. These are actually replicas that were created for Monex to show what the coins look like. They appear nearly identical to the genuine coin (even the hard plastic holder is nearly identical), except the reverse is blank. It is believed that only 1 oz and larger (10 oz and 1 kilogram) sizes were made. Source: http://www.rscott.org/bullion/counter.htm

Sovereigns

You should be aware that in some countries you may find many counterfeits of the 1926 sovereigns. They may be stamped "22" (for 22 karat) on the back, which implies that it is a copy made from real gold (but certainly does not guarantee that!). For more information on counterfeit sovereigns, see Gold Sovereigns from the British Gold Sovereign Specialists.

In the United States, it is illegal to sell counterfeit coins unless the word "copy" is engraved on it. So, you should also look out for that on any coins.

Source: http://www.rscott.org/bullion/counter.htm