Differentiating between childish curiosity and inappropriate sexual interest

As a parent, how can I tell when a child's normal curiosity about sex has crossed over into something inappropriate or unhealthy? My kids are at the age where they're starting to ask questions. That's fine in and of itself, but they've also been showing other signs of interest. For example, I walked in on them “playing doctor” the other day, and it was a pretty awkward experience for all of us.