Answer to old chestnut: light switches
Turn one switch on, then wait several minutes, then turn it off and turn a different one on.
Go to the other room and touch the two bulbs that are not lit. The warm one is controlled by the first switch you touched. The other two are obvious after this.
The assumption is that incandescent light bulbs are used. These days fluorescent lights are common, but many of them don't heat up nearly enough to make this solution
work, or it takes days of the lights being on/off to make a detectable difference in the temperatures.