The best way to determine which country a coin is from is by looking at the text engraved on it. Coins often show the country of origin and the monetary value. Sometimes slogans or other text that can help identify the possible origin.
If the country of origin is not engraved on the coin, determining the language of other text can sometimes indicate the country or region of manufacture. Coin collecting catalogs, which generally have photographs of the coins, can also be consulted. If all else fails, any qualified coin shop or collector should be able to determine the coin's country of origin.