I've known them for.. about 3 years now, and I have no problem pointing out who's who, based on their voices or appearences, took me a few weeks to get to remember everyone but I'm a fast learner
Taeyeon et Seohyun were the easiest for me to point out, based on voice only, as for looks, I had a tendency to mix up seohyun and yuri at first lol
As I first got into them, they were promoting in Japan, so Sooyoung stand out the most as she's the only one who's fluent.. I even thought she was the leader or something at first lol