How can I find out which door has the bad door ajar sensor?

Might try closing all doors, then open one at a time and manually depress the switches. Feel them for "click" feeling and see if by depressing fully you can make the light go out. Remember also that you have a switch which turns the lights to on even with all doors closed. And also, the driver's door sees the most use so that switch is the most likely to fail, plus those hinges wear causing the door to not close as well.