How to spot un-weighted vertices? As you can I see have three rouge vertices that are not a part of a vertex group and/or doesn't have weights on them.



Is there an easy way to spot them without going through each bone?

More clearly: How to spot unassigned vertices in weight paint mode?
Use the options in the armature modifier: "Display modifier in edit mode" and "Adjust edit cage to the modifier result".



In complement, concerned bones are generally the one used for nearly connected vertices, you can see them in the properties panel: