This is really bothering me :P

[Fierlin1182]

I know there's an answer, I just can't remember it.

Three light switches in "OFF" position, linked to three light bulbs upstairs - how can you tell which switch controls which light with only one trip upstairs for inspection?

 

[ve]

Turn one for few minutes,turn it off and turn on another. When you go upstairs you will have one lightening bulb, one worm off and one cold off. I can not think for other answer.

 

[Fierlin1182]

That's it. I knew it was there somewhere.