What language are you using to write your GUI? I'm assuming LabVIEW since you mentioned Front Panel. If this is the case you will need to use an event structure and use a Key down event for This VI. Then you can look at the Mods used. From the LabVIEW help: http://zone.ni.com/reference/en-XX/help/371361N-01/lvprop/control_key_down/
There is other information out there on how to use mod keys and vkeys in the event structure if you google around.
Next question is what kind of List Box are you referring to? Is this an actual TestStand list box or a native to LabVIEW list box? That will change how you select all items.
