edit - to original poster:
The iMac G4 1.25GHz computer shipped with the AHT on the
original OS X installer boot DVD, and is accessible by alternate
boot method keyboard command to run the Test only.
Helps to know your hardware before fielding questions. Free
database download info such as MacTracker is very good; so
is everymac.com and find your model there.
PS: the iMac Software Install & Restore DVD for an iMac G4
17" 1.25GHz computer with superdrive, usb 2.0, etc should
have part number 691-4849-A , DVD version 1.0, AHT version
2.0.3, and shipped with OS X 10.3.2 as the OS version per
the disc set mine came with from Apple.
Hold Opt key to select AHT on startup; or hold C key on start
to use Installer or other options on boot install disc.
Good luck & happy computing! 🙂
{edited to add disc info}