There are books in the library that have pictures that are examples of different grades of coins. The library will be better than the Internet because the variations in grade are so slight that most monitors will not show the difference. If it is a very valuable coin it may be worth having it officially graded, esp. for insurance purposes. Another way could be to find a numismatist group and ask their help. Good Luck!!
To to a grading institution and get it graded, alternatively you could just look in the internet.
This site should help you: http://funadvice.com/r/14j8tgrb9oh