CACHE childcare qualifications can be checked - details plus general guidance on checking job applicants. I would expect that other examination boards offer a similar service to check that a certificate is valid.
A CV is only a guide, it gets someone through the door - it is at the interview that you get to see the paperwork the applicant claims to have.
References can always be tricky, don't rely totally on them. Someone working in childcare for a period of time will typically have built up a portfolio of written references (from families, nurseries, youth organisations etc) which will give you a feel for that person. Yes, it is possible that they are all made up, but your gut instincts I hope will alert you to something fishy.
When interviewing staff, I would say go with your instincts a lot. Also see how they interact with your children, as your children may indicate if they like/dislike a person. Cats can also be a good judge of character, so if you interview someone and your cat decides to lay on their lap, seriously consider hiring them (anyone else find that cats are good at judging someone character).#
Not sure about mothers help wages I'm afraid - though my feeling on mothers help's is that they work along side the mother, so provide shared care, whereas a nanny could provide sole-charge care.