This is a really great topic. I'm convinced I could spot my greys, my GCCs, my sun conure and my gold capped conure based solely on body language, and we've only been parronts for just over a year (less in some cases).
I know their eyes, beaks, behaviors.... but If you were to put my crazy GCC (Rusty) in with 20 other GCCs, how long would it take me to find him? Interesting question... I think I could locate him within a minute.