You could hack two consumer type / household smoke detectors to let your arduino know when they detect smoke
One with an ionization sensor and one with a photoelectric sensor, if they both go off then you almost definitely have smoke present and probably a fire
If I were building a bot like that I would use the two types of smoke detectors, a CO or other gas sensors and a camera and would SMS and send myself a picture and the sensor readings to one or more cell phones and a terrestrial email address
