"While file size is a crude way to do it, there are simple tools based on complex math that will give you precise answers called hash values. How precise you ask?
"Well most people know that everything on a computer is stored in a long string of 1 and/or 0. So every movie, picture or song you have is just a collection of those two numbers. So lets say you have a regular size audio file that is 3 minutes long. We can assume that the file is 3 megabytes in size, so how many 1s and 0s do we have? Each 1 or 0 is a bit, there are 8 bits in a byte, 1000 bytes in a kilobyte and 1000 kilobytes in a megabyte. So how many bits do we have in total? Approximately 24,000,000. If you change the value of any one of those bits then it changes the value of the hash and two files will not be deemed identical. I'd say thats good enough for most people I know :)"
