Is the coin AU or Uncirculated? Sometime it's a tough call.

Begin by checking the fields behind the lettering and date. Can you see the lighter areas behind the U and M in the example above? This is one of the first places to look. Hold a coin in good light and tilt the coin around in different angles to see the surfaces completely. If you see a difference in the protected areas of the field compared to the open areas, the coin is most likely circulated. OR this is also a good way to detect cleaning of a coin as well. If the coin is BU, the fields should be the same in all areas. Wear on fields will often have a gray look.

The major grading services use the term "friction" to describe a type of wear that is not the same as circulated wear. This can be bagmarks and scratches caused by the process of making the coins and delivering them to their destinations of commerce. Also, because most coins are housed together at one time in a roll, they have contact marks. Most gold coins and many Morgan dollars have this friction wear on the high points of the coin. The larger coins and coins with softer metal will have more friction wear.

Professional coin graders have made the analysis that there should be a difference between wear that takes place from the mint to your bank, to when the coin actually enters public circulation. This makes sense because once a coin is struck at the mint it would be hard for the coin not to make contact with anything from the die to the bin to the mint bag full of other coins. The coins of the Carson City Mint were hauled for hundreds of dirt road miles in bags by horse drawn wagons.  Once they reached their destination bank should they still be considered uncirculated?

It's best to look at a LOT of coins and determine the difference between about circulated coins and those with friction. Smaller coins from cents up to quarters are not cut much slack for anything that resembles wear. Turn the coin in good light to try to find any place that could be wear. Check the high points, and again, the fields for difference in color or texture of the surface. Look for consistant mint frost. Look for dull areas. Ask others for their opinions. AU coins are sometimes very hard to determine from online photos. Learn the very highest points of each coin type that is first to wear.