I can identify new people mainly to see if they switch out their secondary or not, particularly with M79 or Barret users. It's funny to shoot at someone, watch them react, fire, and miss, and just kinda wait there hoping for a reload before they die.
I'm also convinced active minigunners are new people. I can't see how some minigun user can be satisfied with 2 kills and 40-odd deaths despite firing off thousands upon thousands of shots.