How can I carefully find out if a professor's work is well-respected within the department? I am a first-year PhD student interested in working with a professor at my university. I have had two nice meetings with him and liked the project ideas we discussed.

But a student told me that he thought the professor's work was derivative and mostly hyped. I asked my academic advisor (a professor who suggests courses to take, answers questions, etc.), who agreed. The thing that stood out from his answer was: "Yeah. Professors talk about other professors, and ..." Which suggested to me that this was not a solitary opinion. The advisor's point was, without being primed, essentially the same as the other student's: the work often seemed like a rebranding and slight modification of other people's work. FWIW, my academic advisor is the research advisor of the student who first warned me. I didn't tell them what they each said, but I can assume they have talked.

Anyway, now I am wary. How can I better judge for myself, given that I am too junior to honestly know if the work is derivative. The professor is quite young but has been publishing in the top journals/conferences in the field.
First of all, every academic has their own opinion of others' work. Just because a student (or your advisor) thought that someone else's work was not significant does not necessarily mean that it is the truth. Although your advisor cites "others" in relaying his opinions, maybe your advisor and the professor in question hate each other, maybe the department places little importance on the professor's work, etc. On the other hand, note that if the professor is publishing in the top journals, then there are some people who think that your professor's work is great also! The point here is that most of the time, opinions are subjective.

There are a couple of ways that you can judge someone's work, being junior.

The first is to see how their students do after graduation. Look at their recent students and postdocs, and see how many remain in academia. Did they manage to secure good positions? Are they productive with research? These are good indications of whether your professor's work is derivative or original.

Secondly, talk to the older grad students or postdocs. They have been around the field long enough to understand what work is important, but not long enough that they are straightforward with you. Talk to a lot of them, and see if there is a consensus, although it would be difficult to get a fair sample!

But most of all, do what interests you! It is good to have an influential advisor, but unless you really believe in your work, I don't think it matters too much whether your research is important or not.