While cleaning is not recommended, it almost impossible to find older coins that have not been cleaned. It is lack of experience that causes people to ask if a coin has been cleaned. For example, a Morgan silver dollar that is over one hundred years old, has probably had dozens, if not hundreds of different owners. A coin may have been cleaned and re-toned multiple times. Over a period of time silver, a chemically active metal, will tone. Coins like the GSA CC Dollars are usually un-cleaned because they have been sealed in uncirculated condition. The only sure way to know a coin has not been cleaned is to buy it certified by a reputable certification company like, PCGS or NGC.

As a general rule, coins should not be cleaned at all as doing so may end up ruining the coin's numismatic value.  The wisest course is to leave your coin collection untouched and stored in the proper holders.

The reason that cleaning coins is rarely a good idea is that cleaning is apt to alter a coin's surface or toning in an undesirable manner (such as leaving small "pock marks", spots or scratches on the coin's surface) unless great care is taken and the proper cleaning method is selected.

It is a good idea to consult a coin dealer or other coin expert to assist you in deciding whether cleaning a particular coin is a good idea. When in doubt do not clean a coin.

If you decide to clean your coins, here are some pointers to keep in mind to avoid or minimize the chances of damaging them.

Dirt and grime that accumulates on coins can often be safely removed by applying a layer of Vaseline to both surfaces of the coin with a Q-Tip and then gently rubbing these surfaces clean with a very soft lint free cloth.

Soaking a coin in olive oil or mild soapy water is another method that can often safely remove dirt or other substances adhering to a coin.

Always use non-abrasive cleaners such as rubbing alcohol or olive oil to clean coins. Commercial jewelry and metal polishes are usually too harsh. Never, for example use silver tarnish remover to clean silver coins as it will adversely affect a coin's natural toning.

Always rinse the coins with distilled water after cleaning. Tap water should not be used as it contains minerals which may create spots on the surface of a coin. Let coin air-dry after cleaning - never rub a coin dry.  

Tarnished coins should only be cleaned (if at all) by a professional using a process known as "dipping". Do not attempt this process yourself.

Some collectors like to use baking soda as a cleaning method to make a worn coin look shiny and new. This is not recommended as it will generally ruin the numismatic value of a coin.

Never clean touch or rub the surface of any Proof coin. If there is dust on the coin simply blow the dust away gently.

Finally it should be noted that specially made commercial coin cleaners are available at most coin supply shops and online.