Outside the room are 3 switches.
Inside the room are 3 light bulbs.
Each bulb is connected to one of the three switches (each it's own).
You can play around the switches before going in the room as much as you want.
But once you open the door, you have to go in to examine the light bulbs and can't touch the switches anymore.

What's your strategy to find out which switch is for which bulb?