As a current nanny I think is important to speak to the previous employer . Also there needs to be a click . The relationship between parents and nannies has be good So both parties are happy . If you meet someone and don't like her personality (even if she comes with great recommendations) don't hire her . Same goes for nannies if you dislike or feel uneasy about the parents don't jump in .
You are most likely going to be seeing this person for a couple of years so if you feel uneasy from the start chances are you will never learn to trust her.

I hired someone who had worked for a colleague for several years and came highly recommended. It worked out very well.

Very easy. I trust my DD's nanny with my child more than I trust my husband. Seriously. DH is too distracted at times and will take calls/texts when watching our DD and lets her watch TV. Nanny doesn't do either.