How do i know if the defrost timer is bad

Freedge keeps working and does not turn off and even it doesn't cool it self. and that makes us to understand that defrost timer is bad.
as it keep working and keep making ICE and as toimer does not work Heater will not work to defrost the ICE and it blocks your GAS chamber wich leads in to freedge keeps working but no cooling process and gas chamber is chocked up.