One question we were told to ask was "If there's an emergency or the child gets sick, and you are there alone, what do you do?"
We got all sorts of wrong answers, like "Change his diaper"?
The helper should know how to call a doctor or emergency or you for help. She should be confident and have good enough English to talk about what to do. Some of the younger helpers we interviewed just said "Yes, ma'am, yes ma'am" to every question, which meant they didn't really comprehend the seriousness of the question.
We also asked if helpers had baby or childcare experience, and whether that experience was with their own kids, or taking care of someone else's kid. If they had done childcare before, we asked the details -- how old were the children? Were they alone with the child? What did they do? Cook for the child? Take the child to school?
Most people prefer married, 30-something or middle-aged mothers, since they are seen as being a steadier presence. Though, personally, it doesn't matter to me whether an amah is single, married, divorced, etc.