I found this in a few different places around the 'net - hopefully it's helpful:

Learn How to Spot a 16-Bit Application

Because most Windows 3.x-based programs run properly under Windows XP, it?s sometimes difficult to tell 16-bit and 32-bit applications apart.

• Right-click the program's executable file and then choose Properties. If you see a Version tab, it's a 32-bit program.
• Or, if the program is running, press Ctrl+Shift+Esc to open the Windows Task Manager. On the Processes tab, look in the Image Name column for the name of the program's executable file. If any 16-bit programs are running, you'll find an entry for Ntvdm.exe, the virtual DOS machine. Just above it in the list, you'll see indented entries for Wowexec.exe (the Windows on Windows subsystem) and the executable name of each 16-bit program running in that virtual machine.