A necessary question as there is some serious overlap between those 2 sites. I will try to formulate my answer in way, so we can use it as list of criterions for migrating from one site to the other.
You could be skeptical about everything in history. "Did this really happen?" "Is it true that...?" Often answerable by Wikipedia/other sources. History.SE is about explaining developments, of course proving and gathering facts is the main work of a historian, but not his final goal. Historians try to see the bigger picture.
Also, I think both sites would profitate from each other, if questions/answers link to each other for explaining developments vs. proving single facts.
Reasons to migrate to Skeptics.SE:
- Proving/Disproving of facts, existence of historical proofs (Did Holocaust really happen, Did people really walk on the moon, 9/11...)
- Questions about statements of single persons (which mostly do not cause/start/influence historical(politcal/social) developments, perhaps some notes/book written by this person does, but then, just read that book, e.g. Karl Marx)
- the answer will likely be mono-causal, a single but important fact often debunks a conspiracy theory
Reasons to migrate to History.SE:
- the question asks about historical developments/situation (thats what historical science is about, explaining and understanding coherently developments on time scales of years and generations)
- the answer has not to prove/disprove facts, but to name all significant ones & put them in a correct chronological order, explaining development based on a historical(social/cultural/economical) context at that specific time
- the answer will be multi-causal and rather tries to explain rather than to prove, as historical sciences often deal with incomplete information and have to use plausibility and exclusion methods
If most agree with my rough points (not covering every special case), I would suggest to make this community wiki, so everybody can add further points.