When I open a word file that's opened by someone else, I get a message explaining who is locking it.
When I do the same with a pptx file, I just get "Read-Only", but I don't know who is locking it. Is there a way to find out?
If you have access to the PC it is shared from and the PC is running Windows XP or above it is very easy to tell who is accessing the file. Go into administrative tools, computer management, under system tools the third option down is shared folders, and it has an option under it called open files. From here you can see who has what files open and disconnect them if need be. You can also see a lot of other useful info; such as what shares are available from the pc and who has a session open via mapped drive but isn't accessing anything.
You may be able to get a hint of who is locking it by looking at the file permissions.
You can view the file permissions by right-clicking a file, selecting properties, and hitting the Security tab (in Windows 7). You can then see who has permissions do certain things. If only one person has full control, obviously they probably locked the file.
-Hope this helps