Some people think they can judge a player after one game, but that's flawed. Players can bust a gut to impress in their first game, meaning you get a false impression of their effort level and they can play above themselves that one time. Other players can suffer from nerves in their first game, which can lead to writing off a player who will settle down to be a very good player. And that's before looking at things like fitness, sharpness, getting to know team mates and tactics of the new club, and getting used to their surroundings.
I'd never write off a player after one, or even a few games, although I have decided a player is a great signing after one game before, with varying results once they settled into their normal game. Stickroth was one who looked like he had the makings of a star, but rarely applied himself after his trial sub appearance against Rangers. Some players have the talent but not the drive, others the determination but not the ability. It's very difficult to say for certain on limited evidence how successful a player will be for us, so I don't get why so many try to do this instant assessment thing.