Installing OS X on new SSD with 10.8 found online
Use the checksum to validate the integrity of the file.

Sometimes when you download large files, the host will provide an MD5 checksum, which is a hash of the raw file data that you can use to compare and see if any of the data was corrupted, changed, or lost during the download process.

To verify the MD5SUM in OS X, simply open the Terminal and type “md5″ followed by one space, and then drag and drop the downloaded file from the finder into the terminal window. 

Hit return on your keyboard and wait a few moments (The larger the file, the longer it will take). 

The terminal will spit out the MD5 hash for the file, which you can then compare to the once provided by the download originator!