Short and long is determined by the total yards gained on the play, not how long the pass was. So if the ball was passed one yard but the receiver ran for an additional ten for a total of 11 gained yards on the play, it would be a long play - even though technically it was a short pass. The announcer will typically tell the audience how far the play was and you can typically see where the ball is each play, telling you the difference between the last play and this one.