I would propose a slight variation on Konrad's answer.
Allow any question that:
Otherwise is a good fit for SE Q&A format
Can be asked legitimately by anyone who is reading about/studying evolution but never heard of creationism/ID. This criterion is slightly subjective, but can be distilled down to "can the essense of the question be preserved without any mention of ID/creationism".
What is a specific evolutionary explanation of XYZ? (good)
Why is ID not considered a science? (Bad, duh)
Basically, if a question needs an (in Konrad's words) "'intelligent design' framing", it's fair game to be considered off topic.
If it is a question that can (and has) be asked by a typical creationist, BUT can just as legitimately be asked by someone just starting to study evolution who never heard of creationism, then it's on-topic as long as it is otherwise good for an SE site.