How to verify that the ISO I downloaded is bootable before I burn it?

isoinfo can probably tell you if it has the right files to boot if you want quick and dirty. isoinfo -l -i is_it_bootable.iso will list the directory structure so you can check for files a live cd / bootable cd should have.

isoinfo -d -i is_it_bootable.iso will tell you if the CD has an El Torito section. Ubuntu's live CD iso reports:

Eltorito validation header:
Hid 1
Arch 0 (x86)
ID ''
Key 55 AA
Eltorito defaultboot header:
    Bootid 88 (bootable)
    Boot media 0 (No Emulation Boot)
    Load segment 0
    Sys type 0
    Nsect 4
    Bootoff 8F 143

You check the MD5SUM. See https://help.ubuntu.com/community/HowToMD5SUM.

The iso at http://www.ubuntu.com/desktop/get-ubuntu/download is guaranteed by ubuntu to be bootable, a valid MD5SUM therefore asserts that the iso is bootable.

Performing a cryptographic hash verification of the ISO file you downloaded consists of the following steps.

1. Open a terminal and type the following:

   md5sum 
   

   Note: there's a space after the md5sum.

2. Now open Nautilus and browse to the folder containing the ISO file.

3. Drag the ISO file to the open terminal window. This will insert the path / filename of the ISO file into the terminal window.
4. Press Enter in the terminal window.
5. The first part of the output is the MD5 hash of the CD.
6. Go here and find the directory that corresponds to your release and find the file MD5SUMS. Compare the hash of your ISO file to the appropriate entry in that file.

If the two hashes do not match, then your ISO file is corrupt and you will need to download it again.