It's a bit difficult to assess the examples from just a sentence or two, but I'll assume you would include the details if they were posted on the main site ;)
- How do I know if he did...
I'm not sure what sort of answers you would be looking for here - it would probably depend on the what was the thing that he did (or did not do). You'd also need to explain why "Just ask him" won't work.
For instance, there's a big difference between something like, "how do I know if my friend told me the truth about what our other friend did?" vs. "how do I find out who stole my lunch?". The key issue is that you need to identify a goal and interpersonal-related skill or problem that you need help with.
My first example could probably be edited to be on topic by instead asking something like "how do I confront Friend B about the thing that he may have done?". Now the question is focused on resolving the conflict, and you're asking for advice with confrontation when you don't have all the facts.
The second would be more difficult to shape up because "identifying a lunch thief" isn't really an interpersonal skill. You'd need to figure out what interpersonal skills you would be exercising to handle the situation and what your end goal is (have your boss / teacher deal with it? talk to the thief and ask why they keep stealing it? etc.)
- It is possible to gain the skill to get whatever you want from others, no matter who he or she is? And if yes, how can I gain it?
Also very broad, but it sounds like it might fall under the category of "social engineering"? There was a meta post about that back when the site had just started, but I don't think we've actually had any questions about it, so I'm not sure if the community still feels the same.
A kind of related topic might be "pickup", and the top answer there suggests that the subject matter is not appropriate for our site as it would violate the principles in our Code of Conduct.
If that's not the sort of thing you meant - it might be a "How do I convince X to do Y" style question. That post gives some good examples of how to rephrase questions to focus on improving your own skills rather than forcing someone else to change.
So again, I think the acceptability of this example would really depend on what your end goal is.
If you're interested in turning either of these into post-able questions, we have a number of resources you can check out! From least to most detailed:
- How To Ask, the standard Stack Exchange protocol.
- What topics can I ask about here?, the help center page that we've customized with IPS-specific guidelines on what is and isn't on topic here.
- How do I write a good question?, our faq-proposed entry for writing questions, which lists elements that should be included with your questions
- The Sandbox, where you can share and revise a draft question before posting on main
- And last but not least, chat is often willing to discuss whether a topic sounds like it could work for the site and even brainstorm some ideas