We have cameras in our home, ostensibly to watch our child whilst we are at work, but they have been helpful in terms of picking up some issues relating to the care of our child. Firstly, I noted that when my child asks for fruit and the helper has been sweeping, cleaning in the bathroom there is no hand washing before the fruit is prepared. This is despite the fact that I have reiterated the importance of this many, many times. Also it is usually hard to determine, because unless you wish to hang out in the kitchen whilst your dinner is being prepared, you can't really know if hands are being washed (after like blowing your nose) and more often than not there are not (probable similar to what happens in restaurants here right?). Now my helper knows that I can see it, she does try a little bit harder but did seem resentful when I brought it up.
Secondly, and more importantly, we have huge windows and are on a high floor with no window grills. I have asked the helpers never to open the windows in the house, yet I knew they were because the window handles were always in different positions when I checked. As the helper is not with my toddler 100% of the time, as I am not when I look after her (one may duck out to get something to eat from the kitchen, answer the front door ect), I considered it a significant safety risk, despite their claims that they always were with my child. After evidence that the windows were indeed being opened, I sorted the problem once and for all and had locks installed on all the windows and took away all the keys, so that's the end to that.
End of the day, you can not be sure that the way you wish something to be done is done to your specifications, and some rules are in place for a good reason even if others (my husband included, he is local and lived in high rises with no grill windows all his young life and he's still alive!!!) believe that you are being over cautious, and won't follow them. A camera can help you mitigate these risks, or when you find out about the problem, at least manage them.
Finally, one further comment, at the end of the day I am generally happy with my helpers, whom I trust and whom I think are kind hearted BUT I do think that most mums who leave their children with helpers or any carer tend to actively look for reasons to trust (self satisficing) as they have to, because at the end of the day, if you don't talk yourself into believing that your helper/carer is trustworthy than you really would be the worst sort of mum, deliberately leaving your child in the hands of someone who could/would harm them.