Notepad++ visibility for BOM
I don't know what you want to achieve so this may be irrelevant, but you could always use a hex editor to see and edit the raw representation of your file.
As far as I know, you can't show/visualize the BOM in Notepad++.
The menu View > Show Symbol > Show All Characters doesn't include the 0xFEFF character.
Following up from Abdull's comment above was able to view ff fe or ÿþ as the Unicode big endian (BE) byte order mark in the N++ hex plugin for N++ (v6.88). Once the plugin is installed click Plugins > View in Hex.