Download and unzip handle.exe to a folder that is in your PATH.

When run without parameters, handle.exe will list the values of all the handles in the system that refer to open files.

When you type handle /?, you receive:

Handle v3.10                              Copyright (C) 1997-2005 Mark Russinovich                              Sysinternals -                              usage: handle  \[-c \] | \[-s \[-p |\] \[name\]                                -a      Dump all handle information.                                -c      Closes the specified handle (interpreted as a hexadecimal number).                                        You must specify the process by its PID.                                        WARNING: Closing handles can cause application or system instability.                                -s      Print count of each type of handle open.                                -u      Show the owning user name when searching for handles.                                -p      Dump handles belonging to process (partial name accepted).                                name    Search for handles to objects with <name> (fragment accepted).                              No arguments will dump all file references.
NOTE: The name parameter is case sensitive and the fragment specified can be anywhere in the file/folder path.