After 15+ years off/on of online playing, I am pretty good at reading players.
It only takes attention.
If I am bored, watching a TV show while playing a freeroll, or trying to play too many games at one time (like I presume wannabe online pros are doing, judging by most of the recent "pro" tips vids I have seen) , then you can only play the cards, not the players, which is at least half the game.
I don't use any HUD or tracker software (BS, IMHO), and I dissuade anyone from going that route. Learn the game, there are no shortcuts.