Considering a presentation is for a group of academics whose specialties are to some (unknown) extent related to the topic, how much of the talk should be spent on explaining the preliminaries?
I often have this problem when preparing slides for a talk where I know that my audience is a group of professors, postdocs, or/and graduate students in the same field or related fields, but I don't know how much background they have on the topic of discussion. The preliminaries are usually topics that could be covered in an upper-level undergrad course (or a first course on that specific topic), but are not a core topic in the field. I know that there could be academics out there who are either not acquainted with the topic or they need a refresher for their knowledge of the material, but I also know that with a very high probability, a considerable number of audience are so comfortable and deeply familiar with the topic that the preliminaries would be extremely boring and trivial for them (and I'm even afraid that it might sound offensively easy to them, especially when it is presented by a student).
Should I just skip the slides on prelims in such cases, or should I always consider the possibility that some members among the audience may be unfamiliar with the topic and the presentation would sound completely meaningless to them if I skip the prelims? I've seen both approaches in academic talks given by experienced professors, so I'm confused about how they decide when to choose which approach.