Check out normal coins
First of all you need to know what normal coins look like. Take coins from your pockets, cars or house and gather them together. Carefully examine each coin especially its front design, back design and edges. Look what is written on these coins. Most coins have imprinted their country of origin, date and marks.
Suspected error coin
When you find a coin which you think it has an error then take a closer look on it. Take the suspected coin and put it side by side with another coin of same value. By this you will be able to differentiate both coins. Some evident features of error coins are printing not in the centre of the coin, coins stamped more than one time, coin blank, coin stamped partially or stamped on a different planchet.
Check the edges
Next you need to check the edges of the coin in order to find an error coin. If the coin has different edges as compared to normal coins of same denomination then it is an error coin. Remember that nickels and pennies have mostly smooth edges whereas quarters and dimes have something carved or milled on the edges.
Use a magnifying glass
Next you need to use a magnifying glass in order to check the lettering, mint mark or date on a coin. In case an image is stamped under letters, letters are misprint or there is a ghost image of the characters then the coin can be a double die error.
Check features with magnifying glass
Sometimes, errors cannot be seen through naked eye so a magnifying glass can be used to check the features of a coin. Coins may have a little error e.g. buffalo stamped on a coin has three legs.