[Nyororin]

I`m going to make the guess that your son is small enough to require her to do physical things with him - Changing clothes, diapers, etc... So I think it would be very very hard to conceal something on him that she would not find. For that length of time, you`ll need a fair sized battery pack and there is no easy way to hide even a tiny one on a small child!

I would suggest maybe hiding a small audio recorder in something you know will be with them through the day. Maybe sewn into the lining of a bag holding his things, or hidden in a part of a stroller, etc.

But really, if there is something about your nanny that leaves you feeling uncomfortable about the care she is giving your son... I wouldn`t look for proof but rather confront her about it and put your concerns out on the table. If she is a truly good care taker then she should want you to feel comfortable with the care she is giving and should work with you to change things for the positive. If she freaks out about listening to some of your concerns, even if there is no abuse going on... she is probably not the type of person you want caring for your son.

And if she is a really good care taker who would be perfectly happy to change her style to fit yours, she would most likely be very hurt and insulted that you jumped straight to surveillance without even trying to talk about the issue.
Clearly there is something bothering you - there shouldn`t be, so I would go straight to her and bring it up. You`re the one paying her, you shouldn`t need to sneak around.