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

How can I check that the ISO image I've 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