How permanently?
I mostly work on theater shows with a run of about one month, which means tagging them permanently isn't really necessary. For longer runs, you might want to consider
e-tape or
gaff tape around the
yoke or
plug.
Some television studios and theme parks I've worked with will use white
paint pen on the body of the
fixture, large enough to be legible from the studio floor. That wouldn't ride in most theaters, though.