I usually look at simple things.
A weak player is often easy to spot because they call too much, fold too little, and are willing to go all-in with just one pair. These players often play almost every hand, ignore position, and like to chase draws without the right odds.
It is also noticeable when someone plays emotionally — after losing a pot, they start playing more hands and try to win it back immediately.
In live games, behavior and table talk help a lot, while online, showdowns and notes are more useful. Often after just a few orbits, it becomes clear who is making weak decisions at the table.