10.6. I can't tell whether Enigmail works or not

If installation was successful, restart Thunderbird / SeaMonkey. Then look on the Mail/News window, which should have an OpenPGP menu on the menubar.
Selecting OpenPGP → About OpenPGP will display the Enigmail version number and GnuPG executable details.

Remember that Enigmail has only been tested with milestone releases of Thunderbird and SeaMonkey. If you use a nightly build, or your own build, Enigmail might not work and might even crash.