Sure! Here's a method found in a book:
You use a pyro electric sensor, and just modify it a little for detecting a small fire. A good sensor would be the PIR sensor you can order from parallax. All I think you do is narrow its field of view (like put a tube around it or something). This way, the sensor will detect smaller fields of Infrared emission, such as that of a candle fire, and not mistake it (hopefully) for something else (cats, squirrels, people, you get the idea).
What you do is mount the sensor on your robot, and have the robot roam around. Have the robot stop every once in a while so it can watch for the the flickering flame of the candle. Otherwise, if the sensor is taking a reading while the robot is in motion, the sensor will read the changing of IR emissions in the environment as the robot is moving. The PIR sensor is compatable with all Parallax microcontrollers, so you might want to go with one of those for the robots processing unit.
(By the way, if you can, I'd get a copy of "Robot Builder's Bonanza 3rd Edition." You can order it from amazon. I wouldn't have been able to get so far in robotics without it!)