Visit them when they have minded children present - they (the children, not the minder) will probably play up horrendously of course, but you should be able to gauge that the children are happy to be with the minder. See how she is with your baby when you visit - she will hopefully ask you questions too. Does she talk to him, and actually acknowledge that he is there?
Ask to talk to ex and current parents they mind for - I suppose you can't guarantee that they aren't just mates with the childminder, but I've sometimes had new parents visit when mindees are being collected, and they will often stay and chat for a few minutes with prospective parent (bit scary!!)
Read their Ofsted report. Ask if they have any written policies you can take home to read (although we will obviously paint ourselves very favourably).
Ask around any friends, neighbours or work colleagues if they have any recommendations.
Ask lots and lots of questions when you visit, and visit as many as you want to - it may help you decide the sort of things you definitely don't want (eg. pack of alsatians loose in garden) and the sort of things you definitely do want (someone who does lots of craft activities).
Ask minders what they would do in a typical day. Ask what sort of things they would do with a baby. Ask if they've had any experience with a "clingy" baby before, and how they resolved it.
Then move to my area because I love a challenge! 