"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 :)"