Reply #45: Short answer: at whoever's shooting at other people who aren't holding a weapon

[Euromutt]

As other posters have noted, this requires some amount of time to observe and evaluate, but I think my short answer makes a decent rule of thumb.

Other possible answers: the person who's changing magazines with the slide of his (or, less frequently, her) autoloader locked back; that's the individual who's already been shooting.
Or, based on other examples of mass shootings: the guy who brought a long gun to a shopping mall. Particularly if it's a cheap semi-auto rifle (SKS variant, AK knockoff or Hi-Point carbine) or pump-action shotgun (Maverick or Mossberg 500-series).