How to ask dumb questions I am having trouble asking questions in seminars, conferences, and public talks.

As a graduate math student I often fail to keep up with speaker and more mature members of the audience at events like seminars and conferences. It is very frustrating to lose track of the talk simply because I am not familiar with one key theorem/definition referenced by the speaker.

On the one hand, many people encourage me to raise my hand whenever something is unclear, motivating it by "no such thing as a stupid question" quote.
On the other, very often I observe people being annoyed by a student asking an "obvious/elementary" question and wasting everyone else's time.

I have seen several related posts on this website, most of which are asked on behalf of the speaker. 
However, my question comes for the opposite side of the barricades. 
I want to know how to find balance between not annoying the speaker and the audience too much on the one hand, and catching up with the talk on the other hand.

More formally, my questions are:


Are there any (semi-, non-) official recommendations on professional etiquette for mathematicians?
I understand that every situation is unique and highly subjective. However, I would be glad if someone gave me advice on when to ask, how to ask, and what to ask at research talks. 
How can I tell if my question is "dumb" (i.e. the answer is well-known or searchable), or if it addresses legitimate ambiguity? 


In particular, how can I quickly determine whether a definition/theorem/lemma mentioned by speaker is a part of common knowledge? 



All relevant links or examples are appreciated.
Keep asking the dumb questions! It is better to look like a fool, than to be one.

You worry that many speakers are annoyed at the `elementary' questions. Some speakers do it because they are stressed about public talking, and any question upsets them. For some, communication of mathematics is not the aim of the talk; they give it because it is a condition of travelling to the venue, or simply because everyone else gives the talks. Instead of feeling joy at an opportunity to clear the confusion, they might get annoyed at having to do the extra work of explaining some of the background. The annoyance has no long-term effect --- nobody holds grudges for asking dumb questions. To give up actually understanding math for such a petty reason is just not worth it.

There is only one situation in which you should refrain from asking a question. That is when you are representative neither of the actual audience nor of the intended audience. So, if you are graduate student at your department's colloquium, it is OK to ask anything. If you are a graduate student at a seminar in your field, it is OK to ask anything. If you are at a seminar in another field, and there are several other students in your field in the room, again it is OK to ask anything. Only if you a lone outsider at a seminar or a conference that is not in your field, there is a reason not to ask questions.