What kind of audience can I expect for a scientific talk at University?
The answer to this question will vary from school to school. Typically speaking, you could expect faculty, staff scientists, post-docs, and graduate students at a generic guest speaker's presentation. As such, you might aim your talk for an audience of educated scientists with familiarity to your field (but not experts of the field). 

If you want more certainty, you could ask the organizer who invited you what audience you can expect. 

If you are comfortable adapting to your audience on the spot, and you are comfortable being somewhat informal, you could ask for a show-of-hands at the beginning of your talk. E.g.: "Show of hands, how many post-docs are there in the audience? ... Graduate students? ... any undergraduates?"