How can I carefully find out if a professor's work is well-respected within the department?
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.