I forget where exactly the tool is, and I don't have pep open in front of me. I think it's under the 2d menu drop down, it's called something like 3D measurement... (my memory is terrible...) I remember the tool doesn't have a button shortcut, and it doesn't have its own icon in the toolbar...
Anyway, it lets you click on 2 different points on the model, and it will tell you the exact distance between those 2 points. So for example, you could click on the 2 shoulder points and you would see what the exact distance between those 2 points would be once the model was complete. It's really helpful for when you're scaling parts.