View > Proof Colors seems to produce no Scripting Listener Code, so I am somewhat skeptical – others may know better, though.
What do you need it for anyway?
A workaround to ensure it is turned it off might be to invoke some random Proof Setup and then toggle Proof Colors.
AppleScript could probably tell you… but thats possibly not what you wanted…?
The reason I need the state of "Proof Colors" is that I want to turn on/off the visibility of a layer set depending on its state.
In the layer set I would have my printing adjustments layers. This way it is very easy to compare the image with the softproof. When "Proof Colors" is turned off the printing adjustment layer set would also be turned off - displaying the image without any printing corrections.
When "Proof Colors" is turned on the printing adjustment layer set would also be turned on - displaying the softproof with printing corrections applied.