1. Working Definition of Performance Art
A performance art piece is an authentic action performed before an audience, framed as performance art, and belonging more readily to the performance art genre than to any other genres.
2. Identifying It: Performance Art is Performance Art
In performance art, a performance art action is framed as performance art, and not as drama, dance, music, "the performing arts", film, video, painting, sculpture, mime, poetry, spoken word, oratory, circus arts, etc., although these and other art elements may be sometimes appear similar to performance art, and they can surely be appropriated by performance art.
Performance art has an art history with lineage and evolution. Mixing two genres together does not necessarily give rise to performance art. However, there is no "usual" means or technology for achieving expression in performance art. Where dance usually employs bodies, film usually employs light, and painting usually employs paint, performance art can employ anything, or re-work the means of other genres to dadaist extremes like bodies through a projector.
More than any other performative art form, performance art is defined by the presence of a performer, an audience, and a performer's action. Dance doesn't require an audience for dancers to dance; drama doesn't need an audience for actors to act. Without an audience, however, performance art ceases, becoming mere action.
Performance art is sometimes compared to drama because both genres employ action, but often they couldn't be more dissimiliar. First, performance art often lacks "acting"—performers may not "pretend" to be characters in other times and places who convey stories. Performance artists may address the audience directly, eschewing the fourth wall. A performance art piece may lack any conscious narrative. There may be no speech, gesture, or movement. If a performance artist makes a voice like a chipmunk, the performer is not necessarily pretending to be, or imitate, a chipmunk—it may simply be the performance artist speaking like a chipmunk, or the chipmunk-speak may have no consciously rational reason for being used at all—performance artists have been known to recite incoherent texts for hours. And when it comes to objects, performance artists may use them as expected, or invent bizarre new uses for them: where an actor might drink from a water glass, the glass to a performance artist might be a car, for example, or a toilet, or the performer's mother, all three, or none.
Here is one test for performance art: if an action can be reframed so it is immediately recognizable as belonging to a historical genre that is not performance art, then the action is probably not performance art.
Here is another: see or do enough performance art, and you'll know what it is (and isn't).
3. Authenticity: Evidence of an Authentic Performance Art Action is Not the Action Itself
A performance art action, as it unfolded, was authentic in space and time. If an action was recorded using technology, the record's contents are not the same as the action that unfolded.
If a record of a performance art action is played or shown, the playing/showing is an action in itself, but this action is not necessarily performance art.
Debris, art objects, and other remnants from a performance art action that unfolded can serve as sculpture, memory, evidence, archeological artifacts, choreographies for future performances, etc., but they are not in themselves performance art.
A record of an authentic performance art action can only simulate the action that unfolded.
Performance art simulations aren't performance art (unless appropriated by performance art).
To witness an authentic performance art action, the audience should be present when and where the action unfolds (unless not having the audience present when and/or where the action unfolds is part of a performance).
Performance art decays instantly and is unrepeatable; because performance art can't be bought or sold like objects, it will always be a radical force in the art world with little to lose.
Performance art manifestoes should never be trusted.