The criteria for inferring design in biology is, as Michael J. Behe, Professor of Biochemistry at Leheigh University, puts it in his book Darwin ‘ s Black Box: “Our ability to be confident of the design of the cilium or intracellular transport rests on the same principles to be confident of the design of anything: the ordering of separate components to achieve an identifiable function that depends sharply on the components.”
ID is based on three premises and the inference that follows (DeWolf et al., Darwinism, Design and Public Education, pg. 92):
1) High information content (or specified complexity) and irreducible complexity constitute strong indicators or hallmarks of (past) intelligent design.
2) Biological systems have a high information content (or specified complexity) and utilize subsystems that manifest irreducible complexity.
3) Naturalistic mechanisms or undirected causes do not suffice to explain the origin of information (specified complexity) or irreducible complexity.
4) Therefore, intelligent design constitutes the best explanations for the origin of information and irreducible complexity in biological systems.
This tells you not only what to look for- the positive case- but also follows Newton and Occam in that if you can slice off the designer by showing that mother nature, father time and their offspring, emergence are all that is required, the design inference is refuted.
Both IC and CSI are examples of work and counterflow. Neither can exist without the intervention of an intelligent agency.
What is irreducible complexity? Wm. Dembski in No Free Lunch, refined the definition as:
IC– A system performing a given basic function is irreducibly complex if it includes a set of well-matched, mutually interacting, non-arbitrarily individuated parts such that each part in the set is indispensable to maintaining the system’s basic, and therefore original, function. The set of these indispensable parts is known as the irreducible core of the system.
Numerous and Diverse Parts If the irreducible core of an IC system consists of one or only a few parts, there may be no insuperable obstacle to the Darwinian mechanism explaining how that system arose in one fell swoop. But as the number of indispensable well-fitted, mutually interacting,, non-arbitrarily individuated parts increases in number & diversity, there is no possibility of the Darwinian mechanism achieving that system in one fell swoop.
Minimal Complexity and Function Given an IC system with numerous & diverse parts in its core, the Darwinian mechanism must produce it gradually. But if the system needs to operate at a certain minimal level of function before it can be of any use to the organism & if to achieve that level of function it requires a certain minimal level of complexity already possessed by the irreducible core, the Darwinian mechanism has no functional intermediates to exploit.