Sitting on the table are 3 light bulbs. 1 of them just blown, 1 of them has been blown for ages, and the other works fine. You don't which is which, but you're allowed to test (turn on) 1 bulb . How can you tell for certain which bulb will work properly.
Hold your hand over each one. The one that's just blown should still be warm.
Try one of the other 2, and you'll know which one works.
NB: I'm assuming several things you haven't explicitly stated (like "test"ing a bulb just means plugging it in).
You carefully break open the light bulbs to examine their filaments. The two blown lightbulbs will have broken filaments, and the just-blown one will have smoke inside the bulb.