First of all Open terminal.
Go to the folder where your downloaded ISO file is located. Use 'cd FOLDER-NAME' command to explore folders/ directories. Replace 'FOLDER-NAME' with correct syntax. Remember that linux is case sensitive.
md5sum FILE-NAME(It may look like = md5sum xubuntu-12.04-desktop-i386.iso )
Then after some seconds a result will show like the line below
Now check that number from corresponding website or supplied text file that if both number is same or not. If both number is matched then your ISO file is OK to burn on a DVD. It is safe to install and explore.
You can apply this method on Ubuntu (all varients), LinuxMint, Peppermint, Edubuntu, Lubuntu, Xubuntu, Kubuntu, antiX, Puppylinux or anyother Linux operating system.