By the size of the dbx file for the folder.

Tools | Options | Maintenance | Store Folder will reveal the location of
your Outlook Express files. Press the Tab key to highlight the folder
location, then Ctrl+C. Close OE, then Start | Run | Ctrl+V will put the
location in the box - Click OK and you'll see the OE files. Otherwise, write
the location down and navigate to it in Windows Explorer.

In WinXP, Win2K & Win2K3, the OE user files (DBX and WAB) are by default
marked as hidden. To view these files in Windows Explorer, you must enable
Show Hidden Files and Folders under Start | Control Panel | Folder Options
Icon | View, or in Windows Explorer | Tools | Folder Options | View.