hey there i hope you can help me with the following.

How can i use a watchdog timer to detect if there is a fault

i have a conveyor belt an i want to say that when my sensor senses a cube then start a timer and if during the time that this timer was counting my sensor didn't sensed a cube again and this timing is now finished then there is a fault. and light a lamp.
How do i impliment the watchdog timer in this?