Strangely enough, after doing the job for 20 years I personally have my own routine and system for checking tickets, which does involve going through after every stop where I can, it actually becomes quite easy to spot those who have just got on, is it infallible? No, is it pretty much effective? Yes.
You learn little things, you learn to read body language, you learn what coffee cups are from where, what part of the train people from certain stops get into and I look out at every station to get an idea of who gets on, I must be pretty effective as my sales/interactions/ticket scans are all as expected and I can't remember the last time I had a complaint that I've had to explain