You're best off asking a professional scout, there's plenty of examples of how scouting works on the internet. But if you had one scout watching Brighton, for example, he could watch them for 46 games, plus preseason, plus cup games. The team might also already have some players on record, so it's a case of watching them and seeing how they have developed (or not, in some cases). It wouldn't take long to see how their prospective stats would have changed.
They don't just watch games, many are watched at training during the day as well, it seems very comprehensive.