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][1] [related][2] [posts][3] 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**:

1. Are there any (semi-, non-) official recommendations on _professional etiquette_ for mathematicians?

2. 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. 

3. 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.

  [1]: http://academia.stackexchange.com/questions/19295/should-i-answer-students-questions-immediately-or-teach-them-to-ask-better-ques
  [2]: http://academia.stackexchange.com/questions/17231/how-to-deal-with-students-asking-very-incremental-questions
  [3]: http://academia.stackexchange.com/questions/17022/useful-strategies-for-answering-dumb-questions-in-a-talk