Flash MX 2004.
shape.isGroup
Read-only property; if true, the shape is a group. A group can contain different types of elements, such as text elements and symbols. However, the group itself is considered a shape, and you can use the shape.isGroup property no matter what types of elements the group contains.
The following example stores the first selected object in the sel variable and then uses the element.elementType and shape.isGroup properties to determine if the selected item is a group:
var sel = fl.getDocumentDOM().selection[0];
var shapeGroup = (sel.elementType == "shape") && sel.isGroup;
fl.trace(shapeGroup);