Ask directly

It is a typical part of the application process. Prospective students want to know what career paths previous students have ended up taking. I would find it concerning if the potential advisor was threatened by this.

As for this concern:

I do not only want to speak to pre-selected students who are going to tell me that everything is awesome when it isn't.

you can ask any students who you are put in touch to tell stories about other PhD students they knew at the time they were there. Such non-vetted students might give you a different picture. That said, PhD students typically end up knowing all of their advisor's strengths and weaknesses, so if you are put in touch with several, you should be able to assemble a good picture of what you are getting in to.

Note that advisors want a good fit with their potential students as well, so all of this should be in their interest too. No need to hide it.

Ask directly

It is a typical part of the application process. Prospective students want to know what career paths previous students have ended up taking. I would find it concerning if the potential advisor was threatened by this.

As for this concern:

I do not only want to speak to pre-selected students who are going to tell me that everything is awesome when it isn't.

you can ask any students who you are put in touch with to tell stories about other PhD students they knew at the time they were there. Such non-vetted students might give you a different picture. That said, PhD students typically end up knowing all of their advisor's strengths and weaknesses, so if you are put in touch with several, you should be able to assemble a good picture of what you are getting in to.

Note that advisors want a good fit with their potential students as well, so all of this should be in their interest too. No need to hide it.