There are a lot of books on grading coins, and many are useful for understanding and identifying the condition of your coins. But most of these books focus on US coins. So what do you do when you have foreign coins? The first thing to know is what the coin looked like when it was first struck. You can not tell how worn a coin is if you have no idea what it looked like brand new. We see this often with people who come in the shop (non-collectors) and they have a coin in G-4 condition but they think it is ”nearly new”, or ”almost uncirculated”. Each coin starts off a bit different and it is helpful to understand the starting point for the coin so you can determine how much wear or contact is on the coin.
Some of the coin books, especially on silver dollars, go into detail on how much detail the coin had to start. Some issues have very soft strikes or had very worn down dies so that the detail was already flat when the coin was struck. All this needs to be taken into account when grading a coin. Many 17th and 18th century coins have either unusual planchets or worn dies. This means you can have a coin with very little detail and still be a very high grade.
So as you learn your coins take note of the starting points on the coins you are collecting.