How can a prospective PhD student know beforehand which universities have the most reputable teams/supervisors?
Most active does not mean most impactful or most influential.  Reading papers on your own, you will have a hard time distinguishing good papers from mediocre work.  Your best bet is to get recommendations from people who already work in that field.  Ask professors at your own university for recommendations or opinions.

If you have a specific problem you are interested in working on, then it does make sense to search for papers on that problem and do your own reading.  But you will still want to get advice from one or more people at your own institution who work in that area (or close to it).