Recognizing a Creation

What constitutes a creation? When do we decide that something is not a haphazard result of chance? We would recognize a computer, a picture of a computer, a row of trees, or a house as something that a human thought about and made. What does SETI look for as a sign of intelligence from other solar systems? In the movie Contact the script writers decided on a set of prime numbers as something that aliens might send in order to let others know that they existed. This would differentiate an intelligent signal for the random, haphazard background radiation in the cosmos.
I think it comes down to probabilities, organization, and patterns. A chair is something we recognize as being a creation. I saw a story of a person who makes shapes out of living trees. He twists, binds, and moves the trees as they are growing to create weird and interesting things. If he made a chair out of living trees in the middle of the forest and someone was to come across it, they would still probably conclude that a person created it although their curiosity would certainly be aroused. Probably no matter how "wild" the weird tree shape guy would make the chair look, people would still conclude that a person made it because the probability of trees growing into that shape by themselves is extremely small. Trees in a row would also result in the same conclusion. You could move every third tree off a random distance or even have a short row of ten trees and we would still conclude intelligent creation. I guess what I'm trying to say is that it takes very little organization, complexity, and pattern before we recognize something as intelligently made. I can't imagine the human body as a result of chance. The probability would be infinitesimally small.