Read papers by possible supervisors - especially papers coauthored with their recent PhD students. Although the reputation of a university is important, much more important is who you'll be spending the next 3+ years of your life working with. Know their stuff! (Obviously you're looking for stuff that makes you think "That's cool" and not "Yuck" - you'll know the former when you see it!) See also this helpful advice by Tara Brabazon on choosing a supervisor.
Read the acknowledgements sections of doctoral theses, ideally from the group you're considering joining, or at least the same department (and building). Beware if they're all one-liners saying, "Thanks to Prof X for being so kind as to talk to me" (you've spent three years with them and still don't use their first name and you don't have anything else to say after all that time?). Better: paragraphs of prose on having a sweet time with a mention or two of the inevitable ups and downs. (Okay, not everybody likes to write this stuff, but there should be one or two in the group.)
Find out what graduates went on to do - are they still around and publishing? They'll have a paper or two coauthored with the supervisor. See what else they published afterwards.
Are there regular research seminars at the dept? This is ESSENTIAL. You need a lively academic community to be stimulated and somewhere to present your ideas. Are there a few well known people around. Do you recognise the names of invited speakers?
Then, if possible (and for a three year commitment it really should be possible): talk to people at the university, ideally in a bar or cafe, to get the gossip!