Download the following files from the mirror:
In a terminal:
$ md5sum aptosid.iso
If they don't match, you will get an error:
md5sum: Warning: calculated checksum does not match!"
If the downloaded file is correct, the program ends without a message.
The manual seems incorrect about this, in multiple languages. The md5sum or sha256sum commands issued like this will only output the checksum and not show if it's correct or not:
$ md5sum aptosid-2011-03-ponos-kde-lite-amd64-201112312100.iso
$ sha256sum aptosid-2011-03-ponos-kde-lite-amd64-201112312100.iso
The command that I know of to compare the checksums is this:
md5sum -c MD5SUM
md5sum: aptosid-2011-03-ponos-kde-full-i386-amd64-201112312100.iso: No such file or directory
aptosid-2011-03-ponos-kde-full-i386-amd64-201112312100.iso: FAILED open or read
I came across this again after I just had a bad checksum on this iso and had to download it again.