This will get a list (return a string array) of the joint names: skinCluster -q -influence "skinName";
This will return the weight, loop through for each joint: skinPercent -t "jointname" -q "skinName" "vertexselectionname";
Items from the skinPercent call that are 0.0 are not influencing the vertex.
Like everything in Maya, there are other ways to do it. You can retrieve an array of values with the -v switch on skinPercent, which could be faster depending on what you are doing.
But I know that method works, and is pretty simple to understand.