What is considered the right way to sign photographs? On the verso or,
why not, on the recto? If you sign it on the verso and have it
mounted, which actually means having it sealed for good, who will ever
be able to know in future times that the photograph has been signed?
If that photograph is sold and resold, how can one be sure that the
back of it contains the signature of the author? Even if the
photograph is accompanied by a certificate of authenticity, which is
rarely the case, that does not guarantee that under the sealed mount
there is a signature of the author, does it? Thank you for any help
you can provide.

There is a few ways to sign your photography.
If you want to sign it for sell or for use it like wallpaper it will
be good to increase canvas and add your sign at bottom. Like National
Geographics do (like this:
http://lava.nationalgeographic.com/pod/pictures/sm_wallpaper/T0499_1.jpg).

Or if you don`t want to sell your photography - put watermarks (not
good way for good photo, because it lose all charm), or put your
name/logo/url/email to not loaded by the important elements angle of
image (like this: http://gallery.photo.net/photo/4633343-md.jpg).

If someone has bought your photo and has resold it someone you have
the right to bring an action against it. And if you have initial
materials of a photo (a film; raw-files; jpg-files of the big
sanction; images removed at the same time and similar to it) can win.
Or if you have already protected your image any license.