I've been hired to teach two courses as an adjunct professor in the US (my first time teaching). I would like to review syllabi previously used for these courses, simply to inform my thinking on these courses. Is this an inappropriate request? It is likely I would ask the department head's administrative assistant for these documents. (As far as I can tell, they are not available online.)
Actually, it is quite appropriate. You could also ask for a list of the professors who previously taught the course. You could then ask them if they are willing to share materials to help bootstrap the course.
In fact, the administrative assistant might not have easy access directly, partly due to the pandemic. But the professors would normally retain their materials.
You might also find recent course information online if each course has a website or if professors have websites with course material.
It is completely appropriate, and can be valuable in the context of the greater curriculum. When I used to teach classes, I would ask the previous lecturer for the syllabus. Most importantly, I would also speak to whomever is teaching a follow-on class to see if there's something that should be added or removed based on the changes to the general curriculum.
As an example, I was teaching a sophomore electrical engineering course as an introduction to analog circuits. I ended up spending a week on the actual device physics because students were going to be required to have either device physics or analog design as a follow-on.
I do the adjunct hiring for my community college department, and we have a certain set of problems that we see repeatedly. One is people teaching a course at the wrong intellectual level (usually too low). Another is not assigning enough work or not giving enough feedback. I would not want a newly hired person to go to an administrative assistant, get a syllabus from one of those people, and then replicate that.
I suggest that you start by talking to the person who hired you and who will be making the decision on whether to rehire you later. Say that you're interested in seeing what people have done in the past when they did a good job of teaching this course. If you said that to me, I would respond by handing you a sample syllabus and also telling you in writing about my expectations. (Well, actually, I would have done those things without your having to ask.)