The Weights Inspector displays the number of weights per vertex by creating a colorized texture (using vertex paint) where each color stands for a specific number of weights (see further down)
The Weight inspector only works on Objects which are bound to an armature. Otherwise the inspector can not determine the weights per bone.
You find the tool in the Avastar Tool Box.
The color code is defined as follows:
More than 4 Bones
(1): The color for unweighted verts can be customized, see User Preferences, Themes Tab, 3D View, Attribute Vertex Group Unreferenced.