1. When you're out on a run and you find yourself trying to identify shoe types by patterns left on the pavement from wet footprints after puddles.
(swear I was behind a Saucony runner the other day, tell-tale triangles on the tread pattern)
2. When you try and (without any training!) do gait analysis on tracks of strangers in the snow based on the length of the "comet trail" left by their foot as the heel comes dragging into the snow (vs. your nice clean footprint with its natural and balanced mid-foot strike of course!)
3. you look to the shoes as runners pass by ("You're running in those?") then to other attributes!