How does one determine when to ask questions?

True. I have no idea if I'm being obvious or obscure, and stop to ask if people are following. You can bisect/triangulate where the dividing line is, I suppose, again by asking. 'Do you mind my asking...?'

What may be good is a few open-ended questions that allows the person to take charge of the conversation, rather than a series of clarifying questions that might come across like a police interrogation.

People actually say I'm a very good listener, although I have to restrain my pedantry, sarcasm and hair-splitting. Friends might be happy or upset about something and I often used to question factual basis of their opinions, which is definitely not what they wanted at that point.

Have you come across the idea of 'active listening'? It's basically paraphrasing what you've heard back to the person.; you may be right or wrong, but it can encourage someone to elaborate until some of the ambiguities are resolved. Can you walk away with unresolved ambiguities that may not be consequential?

It's more guidelines than rules, and the fact that most NTs aren't conscious of them, doesn't mean they aren't learned. They're just learned unexamined and practised so much most people don't need to consciously think what they are.  Autistic girls in particular often learn to blend in by observing what other people do and imitating. Some kind of symmetry where each person talks for about the same proportion of available time is a useful guide in any social situation, less so where there's an 'authority' figure.

Usually certain expressions are only used in a certain context. This is something I'm blind for.

I told a woman once: 'why don't you ask her yourself'... and this was considered a very rude thing... I should have said: 'she still handles these things herself, you should ask her'... 

In that sense usually an NT person would understand the meaning of a certain phrase without mistake, while you might not get it right away. I'm sure you'll find here people willing to help with guessing the right meaning of certain phrases so you can build up your knowledge. 

An extreme example, they'll say: 'get out of here'... and it means: 'that's an unbelievable story, who would have thought.'... 

I learned myself not to go 'really?' when somebody tells me something just to tease me or bully, because I'm very gullible. I also explained this to my son, if somebody told him a story, and he didn't really believe it, just to keep a straight face that could mean anything... a 'yeah, ok...' and then come and check with us, his parents... 

Fact is, if you grasp this better, some people will probably stop talking to you alltogether, but that's more a blessing than a curse. 

These questions are quite difficult to answer because so much depends on the scenario. I'll try though!

Jeffrey said:

How does one know when to just accept what someone says to make them happy, and when to question what someone says?

It might be worth considering how important it is to you to question them. For example, if someone says they love their new green t-shirt and you think it looks blue, it doesn't really matter - it's not going to have a negative impact on either of you if they think it's green. However, if you're told to do something at work and you're not sure you understand what you're supposed to do, you should definitely ask them to clarify. If someone says something that's offensive or goes against your morals (e.g. they make a racist remark), then yes, you can absolutely question them on their beliefs and try to educate them.

Jeffrey said:

How does one determine when to make assumptions about what someone means, and when to ask for clarification on their meaning?

I guess the answer to this question is similar. If it's important for you to be clear on what they mean, ask for clarity. I find it hard to know when to make assumptions too, but there are some scenarios where clarity is really important. For example, my doctor recently told me to take some medication three times a day. If I'd worked on assumptions, I would've assumed that I needed to take them 4 hours apart. However, I asked for clarity on when I should take them, and they said that I needed to take the tablets 20 minutes before a meal. If I hadn't cleared that up, I might have taken them on a full stomach, which would've been the wrong thing to do.

Your last sentence is an example of the bane of my social interactions. "Obvious," is a subjective term, what is obvious to one person is not obvious to another. There is also conflicting data; you say that people usually like good questions, and others have said that people do not like questions. They actually said "too many questions," but again, this is subjective, and thus useless in advice.

It is difficult not to be somewhat vague in my questions because I have trouble remembering exact details of conversations, and those conversations are in the past and (hopefully) won't be repeated, at least not word-for-word. 

I did not actually expect to get rules of conversation that I could use from this forum, but I recognize that I can be wrong, and so I asked the questions anyway. It appears to me that there are no actual rules, and that neuro-typical people have a sort of mind-minding. I know that it is not actual "mind-reading," but that phrase is useful to express my frustration. 

It is too vague to answer but usually people like it when you ask them good questions about what they are talking about or interested in, this is favourable to talking about yourself and own interests to them (unless they ask). Try to avoid asking obvious questions though