How can I tell an adult tooth from a baby tooth when looking in a child's mouth?

Depends on the tooth. If you are looking at the front 4 teeth (incisors), the adult tooth will usually have little ridges on the edge, called mamelons which aid in eruption of the adult teeth. For the adult canines, the angled part of the tooth that makes the point is shorter on the side towards the front teeth and longer on the other side. The baby canines will be opposite. Generally baby teeth are whiter in color.
Looks diferent. Baby teeth are smooth, adult teeth seem a bit serrated and are a different color, they tend to be slightly more yellowish.