Well, the anti-IDers say the bacterial flagellum evolved. What is the test for this?
Science will always assume that an answer can be found
If you can measure something or observe it under consistent conditions then we can say the event is natural. But if an event can't be measured, why is it good science to say one day we will be able to measure it?
Suppose the answer really is that God did it?
The specific history of any feature may never be known. What ID claims is that the flagellum could not have evolved. More specifically, ID claims that component parts of the flagellum genome cannot have any function. This has already been disproved, and should offer some clue as to how this will turn out.
An actual scientist, faced with the problem of the flagellum, would attempt to break the problem down into component problems. Perhaps gene knockouts could demonstrate that simpler versions of the flagellum can work, or that component parts can have useful functions.
This is how science works. Science does not see a problem and throw up its hands.