The way I did it was to get a list from Childrens Information Service (CIS) - I called and they emailed it through. This list contained amongst other details, last date of inspection. I then tallied these dates with the list on the ofsted website. (as there are 365 days in year, I thought chance of being assessed on same day was slim) Why oh why Ofsted or CIS can't combine this information (I think it's about confidentiality of CM), but it is a faff. It took a while, but I got there doing it this way.
Do read between the lines though, as a 'good' childminder may be better for your individual child than the 'outstanding' childminder down the road. (speaking as an 'outstanding' childminder - I know I would be the best )
If you can know where they are and the postcode you can check on the ofsted website, or you put in your postcode and the reports of cm's closest to you will come up, its usually quite easy to work out who they are from the reports.