Another Fun Activity

OK, so two weeks have passed and this was the day that I was supposed to reveal the answer for the last fun activity I did. In my disappointment to see that not even on person replied to the question, I decided that I was not to reveal the answer, in order to encourage the people to reply to it.

I want to give a second question, because I believe this fun activities may help you guys to think critical in order to find the real answer. The reward would be the satisfaction I may feel after deciphering the answer by yourself. So here it goes. Suppose you are at a friend's two story house. On the second floor, there is a windowless room that seems to have no switch to turn on the light. On the outside of the house, there are three light switches, and one seems to be the one that turns on the light of the windowless room on the second floor. Your friend want to see how sagacious you are, so he asks if you can find which of the three switches is the one that turns on the light. You must find the switch without any help, and you are just able to go up to check if the light is on once. No one is going to be up to see if it is on or not. You are alone on this one. What would be your strategy.