And the first step in figuring out whether something is designed is to first understand what nature alone can do without intelligent intervention.<quoted text>
The evidence is the fine toning of the constants of the universe. The design in the universe must be investigated as any other natural phenomena and not simply dismissed.
For example, perfect rectangular solids with open insides are rare to non-existent in nature, so we can know that certain types of buildings are designed simply by their construction.
Or, if we got a signal from space that included a progressing sequence of primes, we would know that an intelligence was behind it since there is no way to produce such signals naturally (that we know of).
Another example can be found in archeology, when we are attempting to determine whether a given rock was modified by intelligence. Do determine that requires knowing how rocks split naturally and finding a difference in the sample from the naturally produced effects.
Now, it is not the complexity of a thing that determines whether it is designed enough. There are many complex phenomena that are perfectly natural and many things that are designed are very simple (example: a paper clip).
So now we come to the universe. First, we know of natural processes acting via gravity that would produce the spiral galaxies, the stars, and the planets. We know there are a wide variety of planets and stars, so it is not at all surprising that a combination like our earth and sun could happen. Once you get a planet in the habitable zone, life is probably fairly easy to get started because the basic materials are well distributed in galaxies. Any 'fine tuning' for the earth specifically is more like the water in a puddle commenting how the earth around it was perfectly designed for the shape of the water. We are a product of our environment, so it isn't surprising that we fit into it. Again, that aspect is perfectly natural.
Now you bring up the 'tuned constants'. It is a good bet that most people who bring up this argument can't say even one of those tuned constants. They are simply parroting what someone else said. But much more relevant is that we do not know how or whether those constants can be different. We simply do not know what nature can do alone. In fact, in a multiverse scenario (and un-verified possibility), it is quite likely that *all* different combinations are found *somewhere*. We just happen to be in a spot that works well for the development of life.
So, your conclusion of design is at least premature until we know what is possible in terms of changing 'constants'. It is even possible that there is some sort of feedback mechanism that forces the constants to be certain values. We simply do not know. And, in some of the theories we have built, it is even the case that all possible combinations happen somewhere, eliminating the design hypothesis from consideration.