That's actually part of the reason ID isn't considered science: there's no possible way to test it. Unless there are some restraints placed on the powers of the hypothetical designer, absolutely any observation is consistent with "Oh, that's just the way the designer did it."
This means that there is no way to either confirm or to falsfy ID.
Evolution theory, on the other hand, places severe limits on what should be and should not be found. EG, the distribution of genetic markers of various types should track the phylogenetic tree: If it's found in both cow and whale genomes, it will also be found in hippos. And so on and so on...
It's unecessary. The existance of an overseeing intelligence in the production of any intelligent design is self evident.
It's there for all to see and people are without excuse for not seeing it.
On the other hand it requires an exercise in willful self deception and blindness to deny ID. Something people can become very skilled at.... if they practice it enough, as evidenced on these boards.
There's no possible way to test the Big Bang theory or Global Warming theories either. But they have been smeared over our science textbooks for years.
Global Warming theorists are like roosters taking credit for the sun rising (they crow every morning, and sure enough the sun rises, it must be them)