Mr. Google defines concise as being an adjective: ‘giving a lot of information clearly and in a few words; brief but comprehensive.’ The more educated audience would likely expect a concise presentation.
Continuing with the prepared speech, creating, with the above point in mind, you need to craft the content of your presentation so that it meets the needs of your audience. If you are writing out your speech notes, you need to always keep in mind ‘is this too wordy?’, “Is there a better way that I can say this?” Rehearsing and delivering practice runs to an audience unrelated to the target audience can be helpful in receiving feedback as to whether you are concise, too concise, or not enough. Getting feedback from a knowledgeable audience is likely the only true way you will ever know. You would then adjust if for future presentations.
As for impromptu speaking situations and speaking concisely, likely having a broad understanding of your topic and related ones, would be helpful. When speaking impromptu, it can be helpful to use a strategy to quickly organize your thoughts. Here is an answer I have written that might help here: How do I improve my extemporaneous speaking skills besides just practicing?
You may find that your audience is not following you, or perhaps an individual will ask you an in depth question i.e. to provide further explanation of a point. You have several choices at this point, each with possible consequences. Ignoring the question or avoiding the question may upset the person who asked the question and upset other members of the audience. Often, if one audience member is wondering abut something, others are as well.
A second response is to answer the question. You have to be prepared that will take time away from your main presentation. If you have allowed for some ‘wiggle room’ as I like to call it, i.e. extra time in your presentation, you should be okay. You need to be aware that questions may have ulterior purposes and take you away from your presentation and your message.
A third method is to acknowledge the question, with the intent to answer it later, at a time of your choosing. You can suggest that you will discuss it in a Q&A (question & Answer) session or perhaps invite the questioner to speak to you after your presentation.
If you are getting a lot of questions related to your content, this may very well be the feedback you are looking for as to whether you are concise enough or too much.
Thanks for your question. I hope I have been concise in my answer.