Use BridgeTalk to communicate with Acrobat, or a shell script.
How to use BridgeTalk? Any idea?
\Unless things have changed with 2015 which I doubt, you can't communicate with acrobat using bridge talk. (Could do up to cs3 or something like that)
Given that you know that if the file is open it is Acrobat that has it open and not a preview in Windows or some other reader then you could try a vbs or applescript snippet.
They would be pretty easy to do.