The theory of evolution states that all organisms are derived from a common ancestor. That is, all modern organisms are genetically related. It's not possible that an organism that uses DNA could be genetically related to one that uses silicon oxides. Be careful not to fall into the false dichotomy fallacy, however. Such an observation would disprove the theory of evolution, but it would NOT necessarily be evidence for ID or creationism (apart from the fact that all observations are consistent with these ideas.)
I also gave that example as just that, an example. There are many other statements that you could use for X when talking about evolution. I also mean the specific theory of evolution that is the current scientific paradigm. I am not saying that a falsification of that theory would lead to another theory that's more to the liking of creationists/ID'ers. A falsification of the current theory of evolution might lead to a new theory that's similar in many ways to the old one. For example, the current theory states that birds and mammals evolved independently from ancestors that were reptiles. Therefore, a statement of the form previously given with X = an organism with both mammal-like and bird-like features would constitute a falsification of the current theory of evolution. If such a falsification were actually found it would lead to a new theory, but it would be a new theory that kept the basics of the current theory with a modified evolutionary tree. This still constitutes a falsification of evolution. If ID'ers were to specify limitations on the capabilities of the designer, such falsifications would be possible for ID as well. That is, if you specified that a designer would not design an eye with a blind spot, that statement would be scientific. It would also be false, since human eyes have a blind spot. It could reasonably be replaced by another ID theory that specifies some other observations that would be impossible.