How to test downloaded "ubuntu-12.04-desktop.iso" md5sum?

Where is the file located in the file system and what is the output of:

pwd

Thanks

cd into the directory that your Ubuntu iso is in using a terminal and type in md5sum <filename>

The file is located in the Documents.What must I type in terminal? Daisy.

cd ~/Documents; md5sum ./ubuntu-12.04-desktop.iso

It solves my problem and I have tested downloaded iso file now and a md5sum is ident as in the official Ubuntu Hashes webpage. But I have another question:I have tested the burned CD too and md5sum is the same as iso file. In the https://help.ubuntu.com/community/HowToMD5SUM is a such advise:"You would simply use a command to get the MD5hash of a burned image: " md5sum /dev/cdrom " .However this will almost NEVER be the same hash as the iso image,that was burned to the disk......"
     Yes,I do so, and the checksum is the same. Is it all right?
      And is it necessary to test every file of the burned CD or not?
   Please,excuse me for my English and thanks very much for Your help! Daisy.

Thanks actionparsnip, that solved my question.