if the light is incandescent or halogen, it will get hot if left on. so switch on the first switch, and get distracted for 10 minutes, switch it off, switch on the next one and race upstairs. If the light is on, it is that 2nd switch. If it is off but hot, it was the first switch. Else it is the third switch. If it's an energy-efficient bulb, this won't work.