CurrentValue can be used to poll the state of numerous system values such as the mouse position. Its help page doesn't list all possible items, though. An item like "TooltipFontFamily" isn't listed but it works nevertheless:

In[49]:= CurrentValue["TooltipFontFamily"] Out[49]= "Segoe UI" 

My question is: where can I find a (more) complete list of items that can be used with CurrentValue. Or, alternatively, how could I programmatically generate such a list?