You test them vs a -10 levels team, so to have all your players at +180% from the opponent which is the max that the simulator simulates, so the new players 4-5* and your 9* will have the same star.
You check the performance, what the player does, basically its about perception and see if the team play fluidm who misses a lot, who is aggressive, who cant score more than 1 goal etc
And for me the best source is the asistant and the academy... players aren't like the old nordgens anymore, but they are a bit better than the ones from the market/scout list.