First turn on one light switch and wait a few minutes until the light bulb would heat up. Then turn that switch off and try and turn on a different one. You then go into the room and check the light bulb. If it is on it was the second switch. If it’s not on check the temperature of the bulb and if it was warm from turning on the first switch for a while then it was the first switch, if not then the one switch you didn’t use was the one switch to turn on the light bulb.