Picky parents are installing hidden cameras and ordering psychological testing to ensure they find suitable nannies.
Private investigator Danny Toreson said his company Paragon had installed two or three hidden cameras to spy on nannies in the past year, but suspected DIY versions were much more common.
Nanny agencies said an increasing number of families were using corporate-style personality testing on job applicants.
But agencies said dodgy nannies were rare and clients were sometimes to blame when things went wrong.
Toreson set up one hidden camera for the parents of two pre-schoolers concerned about their alcohol disappearing when the nanny was there.
"As soon as the parents left, she was straight into the whiskey," said Toreson. The investigators confronted the young woman, who admitted an alcohol problem and resigned.
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