Checking for open files

I have a piece of vba that looks through a folder and pulls information out of a number of workbooks (thanks to a helpful person on here. Apologies that I haven't replied to that thread to say thanks but I can't access the web at the moment and the mobile widget won't let me see my threads. But thanks for the help).

What I'd like is for the system to check the folder for open workbooks and bring up a message box when it finds one, else bring up a message box saying that all workbooks are closed. I can find code to check if a specific file is open, but all I want is to see if any of them are open and for the sub to finish when/if it finds one.