There's no way to test it either. Any result *could* be the hypothetical designer's will. A biologist can reason from the artiodactyl phylogenetic tree that if a genetic marker is found in both pigs and cows it will also be found in deer, giraffes, hippos, and whales. If they aren't there, there is something wrong with the tree or with the underlying theory.
An ID-ist, on the other hand, can only shrug and say "well, maybe the designer put them there, and maybe he didn't". If they're found, it neither confirms or refutes ID; if they're not found, it neither confirms nor refutes ID. What possible use is a "theory" like that?
Ugh. I'm tired of posting the same links again, but here you go.
1. Is Intelligent Design Testable? by William Dembski
2. FAQ: Does intelligent design make predictions? Is it testable?