My only visitors are other parents, occasional friends of my dds when back from university (who all go anywhere except where small children are lurking), and my network co-ordinator (who the children know). I have occasional people visit who might want me as a childminder. I can't remember the last time I had anyone different! I don't invite mates round for coffee.
I will answer my front door to anyone knocking in the same way that anyone would (and am good at getting rid of cold callers!) - I suppose you could argue that this is a "security risk", but I am not going to start getting paranoid.
Lots of childminders now will have some sort of portfolio you can look at - their policies (some are required by Ofsted), a bit of info about themselves, certificates of training etc. Their registration certificate should be on display somewhere, and you should be shown a copy of their last Ofsted inspection.
We get graded, but don't be put off by someone who has been graded as "satisfactory" rather than "good" or "outstanding" - a lot of the grading can relate to paperwork, not interaction with children.
I think you need to get a good gut feeling about whoever you choose. There may be some you like immediately, and others you don't. Don't be afraid to ask questions - about a typical day, how they'd manage your child as well as other mindees, how flexible you might need her/him to be etc. Invite yourself back again if you want, you don't have to decide who you'd like based on one visit.
There's a brilliant list of questions to ask a childminder, and I am sure someone clever will post them soon ...