As a new coin collector, one of the first things you should learn is how to tell the difference between true ancient coins and modern-day fakes. While this article isn't the be-all and end-all on the subject, it will give you an elementary synopsis on authentication to help you avoid deception when getting coins from unusual sources.
Take a good look at the edges of the coin in question. If there is a seam around it, that's an indication that it may be cast. If you notice a flan-crack in the edge, look to see if it goes right through to the other side of the coin. If that flan-crack and/or the edges of the coin is ragged, that's a good sign that you have an authentic ancient coin. If the crack and/or edges of the coin are smooth, that means it was probably cut and isn't as old as you think it is. Flan-cracks that don't go completely through the coin is another sign that this is a cast-coin and not an ancient form of money.
Also look for file marks on the edge of the coin, because one of the first rules of coin collecting is not to do business with anyone selling coins with file marks around the edges It is a very strong sign of a fake coin.
Experienced ancient coin collectors will tell you that the single most important factor in a coin's authenticity is the weight of the coin. It's also the hardest part of counterfeiting coins to get right. Consult the standard reference for the type of coin in question to determine how much it should weigh. Keep in mind that back in the day, coins were typically made of gold and silver, which is much heavier than the metal alloys used in coinage today. If you can't find information on what this coin should weigh, especially if it's expensive, then don't buy it.
Please note: Weight isn't as much of an issue with bronze coins, because it varies greatly from one coin to the next and the metal isn't as valuable.
Grab a magnifying glass and look at the lettering and other fine details to verify that they are well-formed and crisp. Every letter should be uniform in size, and the edges of the letters should have the same wear as the rest of the coin when viewed under magnification. If the coin in question has odd-looking letters, more wear than other parts of the coin, or the letters are not uniform in size, then this is an indication that the coin is a fake.
Artificial patinas are getting better and better each year, making it harder to use patina as a form of authenticating coins. But, artificial patinas still aren't as good as a true patina, making this a good way to help you decide if your ancient coin is real or a fake. A real ancient coin will have a thick patina. Cracks in the patina is a good sign. Sometimes, coin dealers will apply a false patina on a real ancient coin to enhance their beauty. This isn't always a condemnation, but it is preferable to have a true thick patina on an ancient coin.
Thoroughly examine the surface of the coin in question for pitting, especially on the high points of the coin. Pitting is an indication that the coin may have been cast. If you have a gold coin and see small waves in the metal, it's a good sign that the coin is authentic, because those waves are actually the flow lines of the gold metal when the coin was struck by the die.
If the edges of the surface of your coin has some signs of crystallization of the metal, that is another good indication that you have a real ancient coin. However, it can be hard to tell the difference between crystallization and corrosion that occurs when trying to artificially age the coin. If you see crystallization on your coin, take it to an expert to have him or her verify if it's crystallization or corrosion.
These tips will give you a great starting point in deciding if you have an authentic ancient coin or a modern-day fake. You can then use this information to help you decide how much the coin is worth. If you have further questions or still can't determine if you have a fake or real coin, call your local coin dealer. He or she will be happy to help you.