There are plenty of questions here that overlap with politics, religion, art, and so on. How should someone determine if their question would be a better fit on a different Stack than History? This may vary from question to question, but are there any 'blanket rules' that could be applied?
My 2 cents (as a Stack Exchange user with many interests, including the network itself, I'm not a History.SE specialist as my reputation shows):
- A question isn't off-topic on History Stack Exchange because it's on-topic, or even better suited, on another site in the network; it's normal that Stack Exchange sites have some overlap.
- IMHO the best indicator is to ask yourself: what kind of experts would be best suited to answer this question? If it's historians, post it here; if it's politologists (with an interest in political history, of course), post it there. Yes, there will be historians browsing the history section of Politics.SE and vice versa, but less frequently than that they will be on their 'home' site.