You probably already know that rundll32.exe application is used to run or execute .dll files. If you want to find out which .dll file is being executed by a specific instance of rundll32.exe, then it can be done easily.
Start task manager, go to View > Select Columns and select Command Line option.
This will add the full path of the process file being executed in the task manager column. You can find out the .dll file being executed by rundll32.exe from here.