Decide beforehand which "theory" you are going to test. It could be something simple like theory of gravity, or something else that both you and your audience understand very well. I will use gravity for my example here.
You say, "I am going to test the theory of gravity today. I have here some different size and weight of balls and marbles."
Now leave the items and go to the other side of the room.
Say, "It's like I was suspecting, that this gravity thing is not like I thought. People must have been lying. Those balls and marbles aren't doing anything, therefore the theory of gravity is a hoax."
Discuss with your class what is wrong with this picture.
"Testing a theory" means that you get in the middle of it, try it, experience it. Only then can you say with any truth that you believe it or not. It is the same with "testing the gospel." You cannot test it by staying away from church and seeing what comes of that. You test God's word by getting in the middle of it and trying it out and living it.
Source: http://www.geocities.com/Heartland/Flats/4746/lessons.html, additions by Jenny Smith