How do you know who is a vegetarian?

From ANIMAL PEOPLE, June 2004:

A problem inherent in discussing whether Hitler or any
prominent person is or was vegetarian is that people often change
their eating habits–and what they say about their eating
habits–over the course of a lifetime.
In the cases of the Buddha, Isaiah, Pythagoras, Leonardo
da Vinci, Mohandas Gandhi, George Bernard Shaw, and Leo Tolstoy,
ethical vegetarianism was a conscious choice made early in life, and
the public record, though scanty for the earlier figures, affirms
that they were consistent with their professed beliefs.
In the case of consumer advocate Ralph Nader, the public
record has been wildly contradictory for 40 years. Often described
as vegetarian, Nader may never actually have practiced vegetarianism
for any length of time.
But the late labor organizer Cesar Chavez was a conscientious
vegetarian for the latter half of his life. This was rarely
reported, though it was on the record.
In the cases of contemporary entertainment celebrities, the
choice to become vegetarian may be much publicized, yet may be
sustained for no longer than a few days.

Conversely, as in the case of major league baseball manager
Tony LaRussa, longtime vegetarianism at one point appeared to be
belied by a television commercial for a meat product. LaRussa set
the record straight in a fax to ANIMAL PEOPLE: the commercial used a
video clip that he made for a non-meat product, and was soon
withdrawn.
Lacking any better way to establish whether someone is
vegetarian, news media tend to rely upon the person’s own remarks.
But the Daily Telegraph News Service Family Food Panel reported in
June 2004 that only 42% of the British citizens who claim to be
vegetarians totally avoided meat throughout a recent two-week survey
of their eating habits. This was consistent with the findings of the
1992-1993 EAT II survey of Americans, sponsored by the National Live
Stock and Meat Board, and with other studies done in between.
EAT II found that a category of people who do not profess
vegetarianism but simply do not eat meat, called “meat avoiders,”
actually consume less flesh than professed vegetarians other than
vegans, who then and now were about 40% of all people who claimed to
be vegetarian.