The easiest way to check the md5sum for the iso is to navigate into the directory where it was downloaded and right click on a blank spot in the thunar window and click "open terminal here" in the terminal type: "md5sum" followed by a space and then the name of the iso file and hit enter. When it finishes it will display a string of numbers and letters. Check this string against the md5 checksum available on the page you downloaded the iso from. If every thing matches the download is ok, if not you should download the iso again.
“I have not failed. I’ve just found 10,000 ways that won’t work.” - Thomas Edison