How can I query the influences of the Paint Skin Weights Tool?

The active influence can be found in:

global string $artSkinLastSelectedInfluence;

A list of the highlighted influences can be found in:

global string $gArtSkinOrderedInfluenceSelectionList[];

These values are queried\modified inside this script:

C:\Program Files\Autodesk\Maya20XX\scripts\others\artAttrSkinJointMenu.mel

---

The actual control in Maya is a treeView named theSkinClusterInflList. So you can query the selected influences this way:

string $infs[] = `treeView -q -si "theSkinClusterInflList"`;

But this won't tell you the 'lead' influence: Currently it seems you need the above global var to do it.
The ui is made via the proc skinClusterInflBuildList living in this script:

C:\Program Files\Autodesk\Maya20XX\scripts\others\skinClusterInflMenu.mel